diff --git a/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform.rs b/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform.rs
index 6bb731edf..be8d958b9 100644
--- a/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform.rs
+++ b/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform.rs
@@ -94,6 +94,7 @@ impl BindingContext {
     pub fn new() -> Self {
         let mut ctx = BindingContext { bindings: HashMap::new(), names: HashSet::new() };
         ctx.push_new_binding(&Rc::from("static"));
+        ctx.push_new_binding(&Rc::from("unknown"));
         ctx
     }
 }
@@ -154,9 +155,16 @@ impl<'a> LifetimeDefaults<'a> {
     ) -> LifetimeState {
         match lifetime {
             [] => LifetimeState::Unseen,
-            [id] => LifetimeState::Single(
-                self.bindings.get_or_push_new_binding(id, |name| new_bindings.push(name.clone())),
-            ),
+            [id] => {
+                let binding = self
+                    .bindings
+                    .get_or_push_new_binding(id, |name| new_bindings.push(name.clone()));
+                if binding.as_ref() == "unknown" {
+                    LifetimeState::Unknown
+                } else {
+                    LifetimeState::Single(binding)
+                }
+            }
             // TODO(b/454627672): multiple variables.
             _ => LifetimeState::Unknown,
         }
diff --git a/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform_test.rs b/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform_test.rs
index 51ee7c3a7..9546b0978 100644
--- a/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform_test.rs
+++ b/rs_bindings_from_cc/generate_bindings/lifetime_defaults_transform_test.rs
@@ -175,6 +175,37 @@ fn test_unique_lifetime_returned_for_single_ref() -> Result<()> {
     Ok(())
 }
 
+#[gtest]
+fn test_unknown_lifetime_inhibits_default_lifetimes() -> Result<()> {
+    let ir = ir_from_assumed_lifetimes_cc(
+        &(with_full_lifetime_macros()
+            + r#"
+      int& f(int& $unknown i1);
+      "#),
+    )?;
+    let dir = lifetime_defaults_transform(&ir)?;
+    assert_ir_matches!(
+        dir,
+        quote! {
+            Func {
+                cc_name: "f",
+                rs_name: "f", ...
+                return_type: CcType { ... explicit_lifetimes: [] ... }, ...
+                params: [
+                    FuncParam {
+                        type_: CcType { ... explicit_lifetimes: [] ... },
+                        identifier: "i1", ...
+                    }
+                ],
+                ...
+                lifetime_inputs: [],
+                ...
+            }
+        }
+    );
+    Ok(())
+}
+
 #[gtest]
 fn test_no_lifetime_returned_for_distinct_ref_parameters() -> Result<()> {
     let ir = ir_from_assumed_lifetimes_cc(
@@ -775,7 +806,7 @@ fn test_param_lifetimebound_to_this_in_constructor_explicit_lifetime() -> Result
 }
 
 #[gtest]
-fn test_binding_context_has_static() -> Result<()> {
+fn test_binding_context_has_builtin_lifetimes() -> Result<()> {
     let mut ctx = BindingContext::new();
     let mut called = false;
     assert_eq!(
@@ -783,6 +814,11 @@ fn test_binding_context_has_static() -> Result<()> {
         "static".into()
     );
     assert!(!called);
+    assert_eq!(
+        ctx.get_or_push_new_binding(&Rc::from("unknown"), |_| called = true),
+        "unknown".into()
+    );
+    assert!(!called);
     Ok(())
 }
 
@@ -871,6 +907,44 @@ fn test_struct_binds_lifetime_param() -> Result<()> {
     Ok(())
 }
 
+#[gtest]
+fn test_struct_shadows_unknown_lifetime_param() -> Result<()> {
+    let ir = ir_from_assumed_lifetimes_cc(
+        &(with_full_lifetime_macros()
+            + r#"
+      struct LIFETIME_PARAMS("unknown") S { int& $unknown f(); };
+      "#),
+    )?;
+    let dir = lifetime_defaults_transform(&ir)?;
+    assert_ir_matches!(
+        dir,
+        quote! {
+            Record {
+                ...
+                cc_name: "S",
+                ...
+                lifetime_inputs: ["unknown_0"],
+                ...
+            }
+        }
+    );
+    assert_ir_matches!(
+        dir,
+        quote! {
+            Func {
+                cc_name: "f",
+                rs_name: "f", ...
+                return_type: CcType { ... explicit_lifetimes: ["unknown_0"] ... }, ...
+                lifetime_params: [],
+                ...
+                lifetime_inputs: [],
+                ...
+            }
+        }
+    );
+    Ok(())
+}
+
 #[gtest]
 fn test_struct_does_not_shadow_unrelated_lifetime_param() -> Result<()> {
     let ir = ir_from_assumed_lifetimes_cc(
diff --git a/rs_bindings_from_cc/ir_testing.rs b/rs_bindings_from_cc/ir_testing.rs
index bf36c3c0f..97bc43004 100644
--- a/rs_bindings_from_cc/ir_testing.rs
+++ b/rs_bindings_from_cc/ir_testing.rs
@@ -53,6 +53,7 @@ pub fn with_full_lifetime_macros() -> String {
         result.push_str(&format!("#define ${} $({})\n", l, l));
     }
     result.push_str("#define $static $(static)\n");
+    result.push_str("#define $unknown $(unknown)\n");
     result
 }