diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index 416a6b19edfc8..fa3c4cb05f961 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -166,8 +166,9 @@ hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twic
     .label = parameter captured again here
 
 hir_analysis_eii_with_generics =
-    #[{$eii_name}] cannot have generic parameters other than lifetimes
+    `{$impl_name}` cannot have generic parameters other than lifetimes
     .label = required by this attribute
+    .help = `#[{$eii_name}]` marks the implementation of an "externally implementable item"
 
 hir_analysis_empty_specialization = specialization impl does not specialize any associated items
     .note = impl is a specialization of this impl
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 4664bfcce853a..c37611ab2ee70 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -974,12 +974,19 @@ pub(crate) fn check_item_type(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(),
                         (0, _) => ("const", "consts", None),
                         _ => ("type or const", "types or consts", None),
                     };
+                    let name =
+                        if find_attr!(tcx.get_all_attrs(def_id), AttributeKind::EiiForeignItem) {
+                            "externally implementable items"
+                        } else {
+                            "foreign items"
+                        };
+
                     let span = tcx.def_span(def_id);
                     struct_span_code_err!(
                         tcx.dcx(),
                         span,
                         E0044,
-                        "foreign items may not have {kinds} parameters",
+                        "{name} may not have {kinds} parameters",
                     )
                     .with_span_label(span, format!("can't have {kinds} parameters"))
                     .with_help(
diff --git a/compiler/rustc_hir_analysis/src/check/compare_eii.rs b/compiler/rustc_hir_analysis/src/check/compare_eii.rs
index d8afee9aafc98..2beb7eb09c119 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_eii.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_eii.rs
@@ -8,8 +8,9 @@ use std::iter;
 
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Applicability, E0806, struct_span_code_err};
+use rustc_hir::attrs::{AttributeKind, EiiImplResolution};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{self as hir, FnSig, HirId, ItemKind};
+use rustc_hir::{self as hir, FnSig, HirId, ItemKind, find_attr};
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
@@ -169,11 +170,23 @@ fn check_no_generics<'tcx>(
     eii_attr_span: Span,
 ) -> Result<(), ErrorGuaranteed> {
     let generics = tcx.generics_of(external_impl);
-    if generics.own_requires_monomorphization() {
+    if generics.own_requires_monomorphization()
+        // When an EII implementation is automatically generated by the `#[eii]` macro,
+        // it will directly refer to the foreign item, not through a macro.
+        // We don't want to emit this error if it's an implementation that's generated by the `#[eii]` macro,
+        // since in that case it looks like a duplicate error: the declaration of the EII already can't contain generics.
+        // So, we check here if at least one of the eii impls has ImplResolution::Macro, which indicates it's
+        // not generated as part of the declaration.
+        && find_attr!(
+            tcx.get_all_attrs(external_impl),
+            AttributeKind::EiiImpls(impls) if impls.iter().any(|i| matches!(i.resolution, EiiImplResolution::Macro(_)))
+        )
+    {
         tcx.dcx().emit_err(EiiWithGenerics {
             span: tcx.def_span(external_impl),
             attr: eii_attr_span,
             eii_name,
+            impl_name: tcx.item_name(external_impl),
         });
     }
 
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index b388396ac4fcd..185a822bdfa6a 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -1655,10 +1655,12 @@ pub(crate) struct LifetimesOrBoundsMismatchOnEii {
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_eii_with_generics)]
+#[help]
 pub(crate) struct EiiWithGenerics {
     #[primary_span]
     pub span: Span,
     #[label]
     pub attr: Span,
     pub eii_name: Symbol,
+    pub impl_name: Symbol,
 }
diff --git a/tests/ui/eii/type_checking/generic_implementation.rs b/tests/ui/eii/type_checking/generic_implementation.rs
new file mode 100644
index 0000000000000..489fd2e645d84
--- /dev/null
+++ b/tests/ui/eii/type_checking/generic_implementation.rs
@@ -0,0 +1,13 @@
+//@ check-fail
+// Check that type parameters on EIIs are properly rejected.
+// Specifically a regression test for https://github.com/rust-lang/rust/issues/149983.
+#![feature(extern_item_impls)]
+
+#[eii]
+fn foo();
+
+#[foo]
+fn foo_impl<T>() {}
+//~^ ERROR `foo_impl` cannot have generic parameters other than lifetimes
+
+fn main() {}
diff --git a/tests/ui/eii/type_checking/generic_implementation.stderr b/tests/ui/eii/type_checking/generic_implementation.stderr
new file mode 100644
index 0000000000000..17a71998423d7
--- /dev/null
+++ b/tests/ui/eii/type_checking/generic_implementation.stderr
@@ -0,0 +1,12 @@
+error: `foo_impl` cannot have generic parameters other than lifetimes
+  --> $DIR/generic_implementation.rs:10:1
+   |
+LL | #[foo]
+   | ------ required by this attribute
+LL | fn foo_impl<T>() {}
+   | ^^^^^^^^^^^^^^^^
+   |
+   = help: `#[foo]` marks the implementation of an "externally implementable item"
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/eii/type_checking/type_params_149983.rs b/tests/ui/eii/type_checking/type_params_149983.rs
new file mode 100644
index 0000000000000..6dc9b309712e1
--- /dev/null
+++ b/tests/ui/eii/type_checking/type_params_149983.rs
@@ -0,0 +1,10 @@
+//@ check-fail
+// Check that type parameters on EIIs are properly rejected.
+// Specifically a regression test for https://github.com/rust-lang/rust/issues/149983.
+#![feature(extern_item_impls)]
+
+#[eii]
+fn foo<T>() {}
+//~^ ERROR externally implementable items may not have type parameters
+
+fn main() {}
diff --git a/tests/ui/eii/type_checking/type_params_149983.stderr b/tests/ui/eii/type_checking/type_params_149983.stderr
new file mode 100644
index 0000000000000..11f06e6c88e20
--- /dev/null
+++ b/tests/ui/eii/type_checking/type_params_149983.stderr
@@ -0,0 +1,11 @@
+error[E0044]: externally implementable items may not have type parameters
+  --> $DIR/type_params_149983.rs:7:1
+   |
+LL | fn foo<T>() {}
+   | ^^^^^^^^^^^ can't have type parameters
+   |
+   = help: replace the type parameters with concrete types like `u32`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0044`.