diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5b3128a..72cb70d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Prefix the HashiCorp Vault login (both `userpass` and `kubernetes`) with the Vault Enterprise namespace derived from `setup.namespace` when it ends in the KV `data` segment (e.g. `admin/ns/data` → namespace `admin/ns`). Namespace-scoped credentials previously failed authentication with "access denied" because the login always targeted the root namespace. Prefixes whose `data` segment is in the middle (the default and any custom root-namespace subpath) are unaffected and keep logging in against the root namespace.
+
 ### Changed
 
 - Rename the HashiCorp Vault `namespace` setup field label to "Namespace" and clarify its description (namespace and path prefix where parameters are stored).
diff --git a/parameters/providers/hashicorp-vault/README.md b/parameters/providers/hashicorp-vault/README.md
index 0b63413..c413ebc 100644
--- a/parameters/providers/hashicorp-vault/README.md
+++ b/parameters/providers/hashicorp-vault/README.md
@@ -11,15 +11,39 @@ Provider config (from the nullplatform provider specification):
 | Field                  | Required            | Description                                                                 |
 |------------------------|---------------------|-----------------------------------------------------------------------------|
 | `setup.address`        | yes                 | Vault HTTP(S) endpoint, e.g. `https://vault.example.com:8200`.               |
-| `setup.namespace`      | no (default `secret/data/nullplatform`) | KV v2 mount + path prefix parameters are stored under. |
+| `setup.namespace`      | no (default `secret/data/nullplatform`) | KV v2 path prefix parameters are stored under; on Vault Enterprise it may be prefixed with a Vault namespace. |
 | `setup.auth_mode`      | yes (default `userpass`) | Authentication mode: `userpass` or `kubernetes`.                       |
 | `setup.kubernetes_role`| when `kubernetes`   | Vault Kubernetes auth role bound to the agent's ServiceAccount.              |
 
-The KV path prefix defaults to `secret/data/nullplatform` and is configurable via
+The path prefix defaults to `secret/data/nullplatform` and is configurable via
 `setup.namespace` (or the `VAULT_PATH_PREFIX` env var). It must include the KV v2
 `data/` segment. The auth mounts are fixed to Vault's defaults (`auth/userpass`,
 `auth/kubernetes`).
 
+### Vault Enterprise namespaces
+
+On Vault Enterprise, credentials and roles are scoped to a namespace, so the login
+must target that namespace or Vault returns **access denied**. Encode the namespace
+in `setup.namespace` as `<namespace-path>/data` — i.e. the Vault namespace path
+**terminated by the KV v2 `data` segment**. `setup` strips that trailing `/data` to
+recover the namespace and prefixes both the `userpass` and `kubernetes` login with
+it (`store`/`retrieve`/`delete` already reach it because the namespace is embedded
+in the request path).
+
+| `setup.namespace`              | Vault namespace for login | KV path used by store/retrieve |
+|--------------------------------|---------------------------|--------------------------------|
+| `secret/data/nullplatform` *(default)* | *(root, none)*     | `secret/data/nullplatform/…`   |
+| `secret/data/team-x`           | *(root, none)*            | `secret/data/team-x/…`         |
+| `admin/ns/data`                | `admin/ns`                | `admin/ns/data/…`              |
+| `admin/team/sub/data`          | `admin/team/sub`          | `admin/team/sub/data/…`        |
+
+> **Note:** the namespace is derived **only** from a trailing `/data` segment. A
+> prefix whose `data` segment is in the middle (e.g. the default, or a custom
+> subpath like `secret/data/team-x`) is treated as a plain KV mount + path in the
+> **root** namespace — no namespace prefix is added to the login. This keeps
+> non-Enterprise deployments working unchanged. To use a Vault namespace, the
+> prefix must end at `<namespace>/data` (no KV subpath after `data`).
+
 > **Note:** the Vault policy examples below grant access to the default
 > `secret/data/nullplatform/*` (and `secret/metadata/nullplatform/*`) paths. If you
 > set a custom `setup.namespace`, adjust the policy paths to match it — the KV mount
diff --git a/parameters/providers/hashicorp-vault/docs/architecture.md b/parameters/providers/hashicorp-vault/docs/architecture.md
index c49ee6b..c610738 100644
--- a/parameters/providers/hashicorp-vault/docs/architecture.md
+++ b/parameters/providers/hashicorp-vault/docs/architecture.md
@@ -8,7 +8,7 @@ This document describes the `parameters/providers/hashicorp-vault/` implementati
 
 | Step | What happens                                                                          |
 |------|---------------------------------------------------------------------------------------|
-| `setup`     | Reads `VAULT_ADDR`, `VAULT_AUTH_MODE` (default `userpass`) and the mode's inputs, plus `VAULT_PATH_PREFIX` (default `secret/data/nullplatform`). Authenticates against Vault and exports the derived short-lived token as `VAULT_TOKEN`. Fails fast if address or credentials/identity are missing. |
+| `setup`     | Reads `VAULT_ADDR`, `VAULT_AUTH_MODE` (default `userpass`) and the mode's inputs, plus `VAULT_PATH_PREFIX` (default `secret/data/nullplatform`). On Vault Enterprise, derives the Vault namespace from `VAULT_PATH_PREFIX` when it ends in the KV `data` segment (e.g. `admin/ns/data` → `admin/ns`; a `data` segment in the middle is a root-namespace KV path and derives nothing) and prefixes the login URL with it — credentials are namespace-scoped, so a root login would be denied. Authenticates against Vault and exports the derived short-lived token as `VAULT_TOKEN`. Fails fast if address or credentials/identity are missing. |
 | `store`     | Composes the canonical path via `build_external_id` and prepends `VAULT_PATH_PREFIX`, so the `external_id` is the full, self-contained Vault path. POSTs to `$VAULT_ADDR/v1/<full_path>` with a JSON payload. Captures the new version number from Vault's response. Returns `external_id = <VAULT_PATH_PREFIX>/<path>#<version>`. |
 | `retrieve`  | Splits `EXTERNAL_ID` into path + version. GETs `$VAULT_ADDR/v1/<full_path>?version=<N>` using the path **verbatim** (the KV prefix is already embedded — never recomposed against the current `VAULT_PATH_PREFIX`, so a later `setup.namespace` change can't orphan the reference). Returns `{value}` or `{value: "value not found"}`. |
 | `delete`    | Parses path from external_id. DELETEs the metadata endpoint (KV v2) — removes all versions. Idempotent. |
diff --git a/parameters/providers/hashicorp-vault/setup b/parameters/providers/hashicorp-vault/setup
index 1534134..6845704 100755
--- a/parameters/providers/hashicorp-vault/setup
+++ b/parameters/providers/hashicorp-vault/setup
@@ -18,19 +18,53 @@ set -euo pipefail
 # how the token was obtained.
 #
 # The KV path prefix defaults to `secret/data/nullplatform` but is operator-
-# configurable via .setup.namespace (or the VAULT_PATH_PREFIX env var) — it is the
-# KV v2 mount + path prefix, not a Vault Enterprise namespace. Auth mounts are
-# hardcoded to Vault's defaults (auth/userpass, auth/kubernetes). VAULT_ADDR is
-# operator-configurable.
+# configurable via .setup.namespace (or the VAULT_PATH_PREFIX env var). It holds
+# the KV v2 path prefix and, on Vault Enterprise, may be a Vault namespace path
+# terminated by the KV `data` segment: `<namespace-path>/data` (e.g. `admin/ns/data`).
+# The auth mounts are hardcoded to Vault's defaults (auth/userpass, auth/kubernetes),
+# and VAULT_ADDR is operator-configurable.
+#
+# On Vault Enterprise, credentials/roles are scoped to the namespace, so the login
+# must target it too — otherwise Vault returns "access denied". We derive the
+# namespace from VAULT_PATH_PREFIX (see below) and prefix the login URL with it,
+# matching how store/retrieve/delete embed the prefix into their request paths.
+# Prefixes without a trailing `/data` carry no namespace, so login stays on the
+# root namespace (backward compatible).
 #
 # Exports: VAULT_ADDR, VAULT_TOKEN, VAULT_PATH_PREFIX
 
+VAULT_DEFAULT_PATH_PREFIX="secret/data/nullplatform"
+
 VAULT_ADDR=$(get_config_value --env VAULT_ADDR --provider '.setup.address')
 VAULT_AUTH_MODE=$(get_config_value --env VAULT_AUTH_MODE --provider '.setup.auth_mode' --default "userpass")
-VAULT_PATH_PREFIX=$(get_config_value --env VAULT_PATH_PREFIX --provider '.setup.namespace' --default "secret/data/nullplatform")
+VAULT_PATH_PREFIX=$(get_config_value --env VAULT_PATH_PREFIX --provider '.setup.namespace' --default "$VAULT_DEFAULT_PATH_PREFIX")
 # Trim a trailing slash so store/retrieve/delete don't compose a double slash.
 VAULT_PATH_PREFIX="${VAULT_PATH_PREFIX%/}"
 
+# Derive the Vault Enterprise namespace (if any) from the path prefix. We only
+# treat it as namespaced when the prefix ENDS in the KV v2 `/data` segment
+# (e.g. admin/ns/data -> namespace admin/ns): store appends the parameter path
+# after `data`, so the operator configures the prefix up to `<namespace>/data`.
+#
+# A `data` segment in the MIDDLE (secret/data/nullplatform, secret/data/team-x,
+# kv/data/foo) is a plain KV mount + subpath in the ROOT namespace, textually
+# indistinguishable from a namespace, so we must NOT derive one — doing so would
+# break login for existing non-Enterprise deployments. Hence: trailing `/data`
+# only. (Matching on the `/data` suffix is segment-safe: `.../database` does not
+# end in `/data`.)
+VAULT_NAMESPACE=""
+case "$VAULT_PATH_PREFIX" in
+  */data) VAULT_NAMESPACE="${VAULT_PATH_PREFIX%/data}" ;;
+esac
+
+# API base for the login call — namespace-prefixed on Vault Enterprise, plain
+# otherwise. store/retrieve/delete need no equivalent: the namespace is already
+# embedded in VAULT_PATH_PREFIX, which they compose into their request paths.
+VAULT_LOGIN_BASE="$VAULT_ADDR/v1"
+if [ -n "$VAULT_NAMESPACE" ]; then
+  VAULT_LOGIN_BASE="$VAULT_ADDR/v1/$VAULT_NAMESPACE"
+fi
+
 if [ -z "$VAULT_ADDR" ]; then
   log error "❌ Vault address not configured"
   log error ""
@@ -75,7 +109,7 @@ case "$VAULT_AUTH_MODE" in
 
     # URL-encode the username — it is interpolated as a path segment of the login URL.
     VAULT_USERNAME_ENC=$(jq -rn --arg u "$VAULT_USERNAME" '$u | @uri')
-    LOGIN_URL="$VAULT_ADDR/v1/auth/userpass/login/$VAULT_USERNAME_ENC"
+    LOGIN_URL="$VAULT_LOGIN_BASE/auth/userpass/login/$VAULT_USERNAME_ENC"
     LOGIN_PAYLOAD=$(jq -nc --arg password "$VAULT_PASSWORD" '{password: $password}')
     ;;
 
@@ -111,7 +145,7 @@ case "$VAULT_AUTH_MODE" in
     fi
 
     VAULT_K8S_JWT=$(<"$VAULT_K8S_JWT_PATH")
-    LOGIN_URL="$VAULT_ADDR/v1/auth/kubernetes/login"
+    LOGIN_URL="$VAULT_LOGIN_BASE/auth/kubernetes/login"
     LOGIN_PAYLOAD=$(jq -nc --arg role "$VAULT_K8S_ROLE" --arg jwt "$VAULT_K8S_JWT" '{role: $role, jwt: $jwt}')
     ;;
 
diff --git a/parameters/providers/hashicorp-vault/specs/install/hashicorp-vault-configuration.json.tpl b/parameters/providers/hashicorp-vault/specs/install/hashicorp-vault-configuration.json.tpl
index 1bbdf28..01a27d4 100644
--- a/parameters/providers/hashicorp-vault/specs/install/hashicorp-vault-configuration.json.tpl
+++ b/parameters/providers/hashicorp-vault/specs/install/hashicorp-vault-configuration.json.tpl
@@ -52,7 +52,7 @@
           "namespace": {
             "type": "string",
             "title": "Namespace",
-            "description": "Namespace and path prefix under which parameters are stored. Must include the KV v2 `data/` segment (e.g. secret/data/nullplatform).",
+            "description": "Namespace and path prefix under which parameters are stored. Must include the KV v2 `data/` segment (e.g. secret/data/nullplatform). On Vault Enterprise, set it to the namespace path terminated by `data` (e.g. admin/ns/data) — the login targets the namespace and only a trailing `/data` is treated as a namespace.",
             "order": 2,
             "default": "secret/data/nullplatform"
           },
diff --git a/parameters/tests/providers/hashicorp-vault/setup.bats b/parameters/tests/providers/hashicorp-vault/setup.bats
index b75ef59..10b861a 100644
--- a/parameters/tests/providers/hashicorp-vault/setup.bats
+++ b/parameters/tests/providers/hashicorp-vault/setup.bats
@@ -208,6 +208,99 @@ teardown() {
   assert_contains "$captured" "login/ns%2Fagent"
 }
 
+@test "vault setup: userpass login is NOT namespace-prefixed for the default prefix" {
+  export VAULT_ADDR="https://vault.example.com"
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  # default prefix (secret/data/nullplatform) → root namespace, no prefix
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  assert_contains "$captured" "https://vault.example.com/v1/auth/userpass/login/agent"
+}
+
+@test "vault setup: userpass login is prefixed with the Vault namespace (prefix ends in /data)" {
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  # admin/ns/data → Vault namespace = admin/ns
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","namespace":"admin/ns/data"}}'
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  assert_contains "$captured" "https://vault.example.com/v1/admin/ns/auth/userpass/login/agent"
+}
+
+@test "vault setup: VAULT_PATH_PREFIX export keeps the trailing /data segment when a namespace is derived" {
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  # Namespace derivation must not mutate VAULT_PATH_PREFIX itself — store/retrieve/
+  # delete compose their Vault paths from the full prefix (including /data).
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","namespace":"admin/ns/data"}}'
+
+  run bash -c "$DEPS; source $SCRIPT && echo PREFIX=\$VAULT_PATH_PREFIX"
+
+  assert_equal "$status" "0"
+  assert_contains "$output" "PREFIX=admin/ns/data"
+}
+
+@test "vault setup: userpass login is NOT namespace-prefixed for a non-default root prefix (custom subpath, no Enterprise namespace)" {
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  # secret/data/team-x is a plain KV mount + subpath in the root namespace — the
+  # `data` segment is in the MIDDLE, so no namespace is derived (backward compat).
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","namespace":"secret/data/team-x"}}'
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  assert_contains "$captured" "https://vault.example.com/v1/auth/userpass/login/agent"
+}
+
+@test "vault setup: userpass login is NOT namespace-prefixed when a subpath follows the data segment" {
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  # Only a TRAILING /data derives a namespace; admin/ns/data/foo has a subpath
+  # after data, so it is treated as a root KV path (no namespace prefix).
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","namespace":"admin/ns/data/foo"}}'
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  assert_contains "$captured" "https://vault.example.com/v1/auth/userpass/login/agent"
+}
+
+@test "vault setup: userpass login namespace does not treat 'database' as the data segment" {
+  export VAULT_USERNAME="agent"
+  export VAULT_PASSWORD="pw"
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","namespace":"myns/database/x"}}'
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  # 'database' is not the KV 'data' segment → no namespace derived → root login
+  assert_contains "$captured" "https://vault.example.com/v1/auth/userpass/login/agent"
+}
+
+@test "vault setup: kubernetes login is prefixed with the Vault namespace" {
+  export VAULT_AUTH_MODE="kubernetes"
+  export VAULT_K8S_ROLE="nullplatform-agent"
+  export VAULT_K8S_JWT_PATH="$SA_TOKEN_FILE"
+  export PROVIDER_CONFIG='{"setup":{"address":"https://vault.example.com","auth_mode":"kubernetes","kubernetes_role":"nullplatform-agent","namespace":"admin/ns/data"}}'
+
+  run bash -c "$DEPS; source $SCRIPT"
+
+  assert_equal "$status" "0"
+  captured=$(cat "$CURL_LOG")
+  assert_contains "$captured" "https://vault.example.com/v1/admin/ns/auth/kubernetes/login"
+}
+
 @test "vault setup: userpass login without a token fails" {
   export VAULT_ADDR="https://vault.example.com"
   export VAULT_USERNAME="agent"