diff --git a/compiler/rustc_attr_parsing/src/attributes/diagnostic/on_type_error.rs b/compiler/rustc_attr_parsing/src/attributes/diagnostic/on_type_error.rs
index 89cae4b7c55f1..38c1f9ab6c945 100644
--- a/compiler/rustc_attr_parsing/src/attributes/diagnostic/on_type_error.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/diagnostic/on_type_error.rs
@@ -18,6 +18,8 @@ pub(crate) struct OnTypeErrorParser {
 impl OnTypeErrorParser {
     fn parse<'sess>(&mut self, cx: &mut AcceptContext<'_, 'sess>, args: &ArgParser, mode: Mode) {
         if !cx.features().diagnostic_on_type_error() {
+            // `UnknownDiagnosticAttribute` is emitted in rustc_resolve/macros.rs
+            args.ignore_args();
             return;
         }
 
diff --git a/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.rs b/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.rs
new file mode 100644
index 0000000000000..5bacedcb76493
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.rs
@@ -0,0 +1,9 @@
+//@ check-pass
+
+// Regression test for https://github.com/rust-lang/rust/issues/158628.
+
+#[diagnostic::on_type_error(unknown = "")]
+//~^ WARN unknown diagnostic attribute
+pub struct Foo {}
+
+fn main() {}
diff --git a/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.stderr b/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.stderr
new file mode 100644
index 0000000000000..c8b4aac78d6e1
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-diagnostic-on-type-error-malformed-args.stderr
@@ -0,0 +1,11 @@
+warning: unknown diagnostic attribute
+  --> $DIR/feature-gate-diagnostic-on-type-error-malformed-args.rs:5:15
+   |
+LL | #[diagnostic::on_type_error(unknown = "")]
+   |               ^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(diagnostic_on_type_error)]` to the crate attributes to enable
+   = note: `#[warn(unknown_diagnostic_attributes)]` (part of `#[warn(unknown_or_malformed_diagnostic_attributes)]`) on by default
+
+warning: 1 warning emitted
+