diff --git a/crates/uv-workspace/src/pyproject.rs b/crates/uv-workspace/src/pyproject.rs
index 9caf24ec021af..0de4751330013 100644
--- a/crates/uv-workspace/src/pyproject.rs
+++ b/crates/uv-workspace/src/pyproject.rs
@@ -35,6 +35,7 @@ use uv_pypi_types::{
     Conflicts, DependencyGroups, SchemaConflicts, SupportedEnvironments, VerbatimParsedUrl,
 };
 use uv_redacted::DisplaySafeUrl;
+use uv_warnings::warn_user_once;
 
 #[derive(Error, Debug)]
 pub enum PyprojectTomlError {
@@ -276,10 +277,11 @@ pub struct Tool {
     pub uv: Option<ToolUv>,
 }
 
-/// Validates that index names in the `tool.uv.index` field are unique.
+/// Validates the `tool.uv.index` field.
 ///
-/// This custom deserializer function checks for duplicate index names
-/// and returns an error if any duplicates are found.
+/// This custom deserializer function checks for:
+/// - Duplicate index names
+/// - Multiple indexes marked as default
 fn deserialize_index_vec<'de, D>(deserializer: D) -> Result<Option<Vec<Index>>, D::Error>
 where
     D: Deserializer<'de>,
@@ -287,6 +289,7 @@ where
     let indexes = Option::<Vec<Index>>::deserialize(deserializer)?;
     if let Some(indexes) = indexes.as_ref() {
         let mut seen_names = FxHashSet::with_capacity_and_hasher(indexes.len(), FxBuildHasher);
+        let mut seen_default = false;
         for index in indexes {
             if let Some(name) = index.name.as_ref() {
                 if !seen_names.insert(name) {
@@ -295,6 +298,16 @@ where
                     )));
                 }
             }
+            if index.default {
+                if seen_default {
+                    warn_user_once!(
+                        "Found multiple indexes with `default = true`; \
+                        only one index may be marked as default. \
+                        This will become an error in the future.",
+                    );
+                }
+                seen_default = true;
+            }
         }
     }
     Ok(indexes)
diff --git a/crates/uv/tests/it/lock.rs b/crates/uv/tests/it/lock.rs
index 8fff3c72588e4..aa46e8c1adadb 100644
--- a/crates/uv/tests/it/lock.rs
+++ b/crates/uv/tests/it/lock.rs
@@ -19250,6 +19250,45 @@ fn lock_repeat_named_index() -> Result<()> {
     Ok(())
 }
 
+/// If multiple indexes are marked as default within a single file, we should raise an error.
+#[test]
+fn lock_multiple_default_indexes() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+
+        [[tool.uv.index]]
+        name = "first"
+        url = "https://pypi.org/simple"
+        default = true
+
+        [[tool.uv.index]]
+        name = "second"
+        url = "https://example.com"
+        default = true
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Found multiple indexes with `default = true`; only one index may be marked as default. This will become an error in the future.
+    Resolved 2 packages in [TIME]
+    ");
+
+    Ok(())
+}
+
 /// If a name is defined in both the workspace root and the member, prefer the index in the member.
 #[test]
 fn lock_repeat_named_index_member() -> Result<()> {
@@ -33274,6 +33313,7 @@ fn lock_check_multiple_default_indexes_explicit_assignment_dependency_group() ->
     ----- stdout -----
 
     ----- stderr -----
+    warning: Found multiple indexes with `default = true`; only one index may be marked as default. This will become an error in the future.
     Resolved 2 packages in [TIME]
     ");
 
@@ -33324,6 +33364,7 @@ fn lock_check_multiple_default_indexes_explicit_assignment_dependency_group() ->
     ----- stdout -----
 
     ----- stderr -----
+    warning: Found multiple indexes with `default = true`; only one index may be marked as default. This will become an error in the future.
     Resolved 2 packages in [TIME]
     ");