diff --git a/Cargo.lock b/Cargo.lock
index 5d7f296788fcc..126e0d716ce52 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -5927,6 +5927,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "temp-env",
  "thiserror 2.0.17",
  "toml",
  "tracing",
diff --git a/crates/uv-auth/src/lib.rs b/crates/uv-auth/src/lib.rs
index e1fe92f17ab59..7820ed1ed3be1 100644
--- a/crates/uv-auth/src/lib.rs
+++ b/crates/uv-auth/src/lib.rs
@@ -44,6 +44,16 @@ pub(crate) static CREDENTIALS_CACHE: LazyLock<CredentialsCache> =
 pub fn store_credentials_from_url(url: &DisplaySafeUrl) -> bool {
     if let Some(credentials) = Credentials::from_url(url) {
         trace!("Caching credentials for {url}");
+
+        // If credentials already exist in the cache for this URL, do not override them
+        // with URL-derived credentials. This ensures credentials provided via environment
+        // variables (or other higher-precedence sources) are not replaced by credentials
+        // embedded in an index URL.
+        if CREDENTIALS_CACHE.get_url(url, &Username::none()).is_some() {
+            trace!("Skipping caching credentials for {url}: credentials already present");
+            return false;
+        }
+
         CREDENTIALS_CACHE.insert(url, Arc::new(Authentication::from(credentials)));
         true
     } else {
@@ -58,3 +68,32 @@ pub fn store_credentials(url: &DisplaySafeUrl, credentials: Credentials) {
     trace!("Caching credentials for {url}");
     CREDENTIALS_CACHE.insert(url, Arc::new(Authentication::from(credentials)));
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_store_credentials_from_url_does_not_override_existing_cache() {
+        use crate::credentials::Credentials;
+
+        let base = DisplaySafeUrl::parse("https://example.com/simple/real").unwrap();
+        // Seed the cache with env-var-like credentials using store_credentials
+        let env_creds =
+            Credentials::basic(Some("envuser".to_string()), Some("envpass".to_string()));
+        store_credentials(&base, env_creds.clone());
+
+        // Construct a URL with embedded credentials that would otherwise override the cache
+        let mut url_with_creds = base.clone();
+        let _ = url_with_creds.set_username("urluser");
+        let _ = url_with_creds.set_password(Some("urlpass"));
+
+        // Attempt to store credentials from the URL; expect we will not override existing env creds
+        assert!(!store_credentials_from_url(&url_with_creds));
+
+        // Ensure the cached credentials are the original env creds
+        let cached = CREDENTIALS_CACHE.get_url(&base, &Username::none()).unwrap();
+        assert_eq!(cached.username(), env_creds.username());
+        assert_eq!(cached.password(), env_creds.password());
+    }
+}
diff --git a/crates/uv-distribution-types/Cargo.toml b/crates/uv-distribution-types/Cargo.toml
index 4963cac18f9bf..000e91f62e34a 100644
--- a/crates/uv-distribution-types/Cargo.toml
+++ b/crates/uv-distribution-types/Cargo.toml
@@ -53,6 +53,7 @@ version-ranges = { workspace = true }
 
 [dev-dependencies]
 toml = { workspace = true }
+temp-env = { workspace = true }
 
 [features]
 schemars = ["dep:schemars", "uv-redacted/schemars"]
diff --git a/crates/uv-distribution-types/src/index.rs b/crates/uv-distribution-types/src/index.rs
index 2090f8ba85a5b..bfe1c561b3aa0 100644
--- a/crates/uv-distribution-types/src/index.rs
+++ b/crates/uv-distribution-types/src/index.rs
@@ -6,6 +6,8 @@ use thiserror::Error;
 use url::Url;
 
 use uv_auth::{AuthPolicy, Credentials};
+// Avoid importing `uv_static` here as it's not a dependency of this crate. We construct the
+// environment variable names directly using the same pattern as `uv_static::EnvVars`.
 use uv_redacted::DisplaySafeUrl;
 use uv_small_str::SmallString;
 
@@ -346,15 +348,54 @@ impl Index {
 
     /// Retrieve the credentials for the index, either from the environment, or from the URL itself.
     pub fn credentials(&self) -> Option<Credentials> {
-        // If the index is named, and credentials are provided via the environment, prefer those.
+        // Extract credentials from the index URL, if any.
+        let url_credentials = Credentials::from_url(self.url.url());
+
+        // If the index is named, and credentials are provided via the environment, we should
+        // prefer any provided environment variable values. Specifically, environment variables
+        // should override the username in the index URL. We also merge values when possible,
+        // such that an environment-provided password can be combined with a URL-provided
+        // username (and vice-versa), while environment-provided values take precedence.
         if let Some(name) = self.name.as_ref() {
-            if let Some(credentials) = Credentials::from_env(name.to_env_var()) {
-                return Some(credentials);
+            let name_env = name.to_env_var();
+            let username_env_key = format!("UV_INDEX_{name_env}_USERNAME");
+            let password_env_key = format!("UV_INDEX_{name_env}_PASSWORD");
+
+            let username_present = std::env::var_os(&username_env_key).is_some();
+            let password_present = std::env::var_os(&password_env_key).is_some();
+
+            if username_present || password_present {
+                // Read the actual environment values (may be empty string).
+                let username_env = std::env::var(&username_env_key).ok();
+                let password_env = std::env::var(&password_env_key).ok();
+
+                // If the environment provided a username, use that. Otherwise, if the URL
+                // provided a username, use the URL's username. If neither provided one, the
+                // username is None.
+                let username = if username_present {
+                    username_env
+                } else {
+                    url_credentials
+                        .as_ref()
+                        .and_then(|creds| creds.username().map(ToString::to_string))
+                };
+
+                // If the environment provided a password, use that. Otherwise, if the URL
+                // provided a password, use the URL's password.
+                let password = if password_present {
+                    password_env
+                } else {
+                    url_credentials
+                        .as_ref()
+                        .and_then(|creds| creds.password().map(ToString::to_string))
+                };
+
+                return Some(Credentials::basic(username, password));
             }
         }
 
         // Otherwise, extract the credentials from the URL.
-        Credentials::from_url(self.url.url())
+        url_credentials
     }
 
     /// Resolve the index relative to the given root directory.
@@ -603,4 +644,69 @@ mod tests {
         assert_eq!(cache_control.api.as_deref(), Some("max-age=300"));
         assert_eq!(cache_control.files, None);
     }
+
+    #[test]
+    fn test_index_credentials_env_username_overrides_url_username() {
+        use crate::IndexUrl;
+        use temp_env::with_vars;
+        const USERNAME_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_USERNAME";
+        const PASSWORD_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_PASSWORD";
+
+        // Set env var username and clear password env var using with_vars to ensure safe testing
+        with_vars(
+            vec![(USERNAME_ENV, Some("envuser")), (PASSWORD_ENV, None)],
+            || {
+                // Index URL includes credentials
+                let url = IndexUrl::from_str("https://urluser:pass@example.com/simple").unwrap();
+                let mut index = Index::from(url);
+                index.name = Some(IndexName::new("private-registry").unwrap());
+
+                let creds = index.credentials().unwrap();
+                assert_eq!(creds.username(), Some("envuser"));
+                assert_eq!(creds.password(), Some("pass"));
+            },
+        );
+    }
+
+    #[test]
+    fn test_index_credentials_env_password_merges_with_url_username() {
+        use crate::IndexUrl;
+        use temp_env::with_vars;
+        const USERNAME_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_USERNAME";
+        const PASSWORD_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_PASSWORD";
+
+        with_vars(
+            vec![(USERNAME_ENV, None), (PASSWORD_ENV, Some("envpass"))],
+            || {
+                let url = IndexUrl::from_str("https://urluser:pass@example.com/simple").unwrap();
+                let mut index = Index::from(url);
+                index.name = Some(IndexName::new("private-registry").unwrap());
+
+                let creds = index.credentials().unwrap();
+                assert_eq!(creds.username(), Some("urluser"));
+                assert_eq!(creds.password(), Some("envpass"));
+            },
+        );
+    }
+
+    #[test]
+    fn test_index_credentials_env_username_empty_string_overrides_url_username() {
+        use crate::IndexUrl;
+        use temp_env::with_vars;
+        const USERNAME_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_USERNAME";
+        const PASSWORD_ENV: &str = "UV_INDEX_PRIVATE_REGISTRY_PASSWORD";
+
+        with_vars(
+            vec![(USERNAME_ENV, Some("")), (PASSWORD_ENV, Some("envtoken"))],
+            || {
+                let url = IndexUrl::from_str("https://urluser:pass@example.com/simple").unwrap();
+                let mut index = Index::from(url);
+                index.name = Some(IndexName::new("private-registry").unwrap());
+
+                let creds = index.credentials().unwrap();
+                assert_eq!(creds.username(), None);
+                assert_eq!(creds.password(), Some("envtoken"));
+            },
+        );
+    }
 }
diff --git a/crates/uv-git/src/credentials.rs b/crates/uv-git/src/credentials.rs
index 0515609800c11..a381146019d6e 100644
--- a/crates/uv-git/src/credentials.rs
+++ b/crates/uv-git/src/credentials.rs
@@ -32,9 +32,45 @@ impl GitStore {
 pub fn store_credentials_from_url(url: &DisplaySafeUrl) -> bool {
     if let Some(credentials) = Credentials::from_url(url) {
         trace!("Caching credentials for {url}");
-        GIT_STORE.insert(RepositoryUrl::new(url), credentials);
+        // If the store already contains credentials for this repository, don't override them
+        // with credentials embedded in the URL. This respects higher-precedence credential
+        // sources such as environment variables and keyrings.
+        let repo = RepositoryUrl::new(url);
+        if GIT_STORE.get(&repo).is_some() {
+            trace!("Skipping caching Git credentials for {url}: credentials already present");
+            return false;
+        }
+        GIT_STORE.insert(repo, credentials);
         true
     } else {
         false
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_store_credentials_from_url_does_not_override_git_store() {
+        use uv_auth::Credentials;
+
+        let base = DisplaySafeUrl::parse("https://github.com/astral-sh/uv").unwrap();
+        let repo = RepositoryUrl::new(&base);
+        // Seed the store with env-var-like creds
+        let env_creds =
+            Credentials::basic(Some("envuser".to_string()), Some("envpass".to_string()));
+        GIT_STORE.insert(repo.clone(), env_creds.clone());
+
+        // URL with embedded credentials that would have overridden
+        let mut url_with_creds = base.clone();
+        let _ = url_with_creds.set_username("urluser");
+        let _ = url_with_creds.set_password(Some("urlpass"));
+
+        assert!(!store_credentials_from_url(&url_with_creds));
+
+        let cached = GIT_STORE.get(&repo).unwrap();
+        assert_eq!(cached.username(), env_creds.username());
+        assert_eq!(cached.password(), env_creds.password());
+    }
+}