diff --git a/crates/uv-workspace/src/pyproject.rs b/crates/uv-workspace/src/pyproject.rs
index e0821dac30559..c7d365ebd79e4 100644
--- a/crates/uv-workspace/src/pyproject.rs
+++ b/crates/uv-workspace/src/pyproject.rs
@@ -978,6 +978,7 @@ impl Source {
         source: RequirementSource,
         workspace: bool,
         editable: Option<bool>,
+        index: Option<String>,
         rev: Option<String>,
         tag: Option<String>,
         branch: Option<String>,
@@ -1018,7 +1019,19 @@ impl Source {
         }
 
         let source = match source {
-            RequirementSource::Registry { .. } => return Ok(None),
+            RequirementSource::Registry { index: Some(_), .. } => {
+                return Ok(None);
+            }
+            RequirementSource::Registry { index: None, .. } => {
+                if let Some(index) = index {
+                    Source::Registry {
+                        index,
+                        marker: MarkerTree::TRUE,
+                    }
+                } else {
+                    return Ok(None);
+                }
+            }
             RequirementSource::Path { install_path, .. }
             | RequirementSource::Directory { install_path, .. } => Source::Path {
                 editable,
diff --git a/crates/uv/src/commands/project/add.rs b/crates/uv/src/commands/project/add.rs
index 573dd50ed582b..c64664d4e80c4 100644
--- a/crates/uv/src/commands/project/add.rs
+++ b/crates/uv/src/commands/project/add.rs
@@ -366,6 +366,13 @@ pub(crate) async fn add(
         }
     }
 
+    // If the user provides a single, named index, pin all requirements to that index.
+    let index = indexes
+        .first()
+        .as_ref()
+        .and_then(|index| index.name.as_ref())
+        .filter(|_| indexes.len() == 1);
+
     // Add the requirements to the `pyproject.toml` or script.
     let mut toml = match &target {
         Target::Script(script, _) => {
@@ -394,6 +401,7 @@ pub(crate) async fn add(
                     requirement,
                     false,
                     editable,
+                    index.cloned(),
                     rev.clone(),
                     tag.clone(),
                     branch.clone(),
@@ -409,6 +417,7 @@ pub(crate) async fn add(
                     requirement,
                     workspace,
                     editable,
+                    index.cloned(),
                     rev.clone(),
                     tag.clone(),
                     branch.clone(),
@@ -690,7 +699,11 @@ async fn lock_and_sync(
             };
 
             // Only set a minimum version for registry requirements.
-            if edit.source.is_some() {
+            if edit
+                .source
+                .as_ref()
+                .is_some_and(|source| !matches!(source, Source::Registry { .. }))
+            {
                 continue;
             }
 
@@ -881,6 +894,7 @@ fn resolve_requirement(
     requirement: pypi_types::Requirement,
     workspace: bool,
     editable: Option<bool>,
+    index: Option<String>,
     rev: Option<String>,
     tag: Option<String>,
     branch: Option<String>,
@@ -891,6 +905,7 @@ fn resolve_requirement(
         requirement.source.clone(),
         workspace,
         editable,
+        index,
         rev,
         tag,
         branch,
diff --git a/crates/uv/tests/edit.rs b/crates/uv/tests/edit.rs
index f98be17d578be..b2190991b6eca 100644
--- a/crates/uv/tests/edit.rs
+++ b/crates/uv/tests/edit.rs
@@ -5064,7 +5064,7 @@ fn add_no_warn_index_url() -> Result<()> {
 
 /// Add an index provided via `--index`.
 #[test]
-fn add_index_url() -> Result<()> {
+fn add_index() -> Result<()> {
     let context = TestContext::new("3.12");
 
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
@@ -5149,6 +5149,7 @@ fn add_index_url() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Missing version constraint (e.g., a lower bound) for `jinja2`
     Resolved 4 packages in [TIME]
     Prepared 2 packages in [TIME]
     Installed 2 packages in [TIME]
@@ -5178,6 +5179,9 @@ fn add_index_url() -> Result<()> {
 
         [[tool.uv.index]]
         url = "https://pypi.org/simple"
+
+        [tool.uv.sources]
+        jinja2 = { index = "pytorch" }
         "###
         );
     });
@@ -5237,7 +5241,7 @@ fn add_index_url() -> Result<()> {
         [package.metadata]
         requires-dist = [
             { name = "iniconfig", specifier = "==2.0.0" },
-            { name = "jinja2", specifier = ">=3.1.3" },
+            { name = "jinja2", specifier = ">=3.1.3", index = "https://download.pytorch.org/whl/cu121" },
         ]
         "###
         );
@@ -5276,6 +5280,9 @@ fn add_index_url() -> Result<()> {
 
         [[tool.uv.index]]
         url = "https://pypi.org/simple"
+
+        [tool.uv.sources]
+        jinja2 = { index = "pytorch" }
         "###
         );
     });
@@ -5341,7 +5348,7 @@ fn add_index_url() -> Result<()> {
         [package.metadata]
         requires-dist = [
             { name = "iniconfig", specifier = "==2.0.0" },
-            { name = "jinja2", specifier = ">=3.1.3" },
+            { name = "jinja2", specifier = ">=3.1.3", index = "https://test.pypi.org/simple" },
         ]
         "###
         );