diff --git a/tpl/templates/templates_integration_test.go b/tpl/templates/templates_integration_test.go
index 93922b4c4..a0a5e385a 100644
--- a/tpl/templates/templates_integration_test.go
+++ b/tpl/templates/templates_integration_test.go
@@ -1,4 +1,4 @@
-// Copyright 2024 The Hugo Authors. All rights reserved.
+// Copyright 2025 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
diff --git a/tpl/tplimpl/legacy_integration_test.go b/tpl/tplimpl/legacy_integration_test.go
new file mode 100644
index 000000000..a96e35fca
--- /dev/null
+++ b/tpl/tplimpl/legacy_integration_test.go
@@ -0,0 +1,38 @@
+// Copyright 2025 The Hugo Authors. All rights reserved.
+//
+// Portions Copyright The Go Authors.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tplimpl_test
+
+import (
+	"testing"
+
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestLegacyPartialIssue13599(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/partials/mypartial.html --
+Mypartial.
+-- layouts/_default/index.html --
+mypartial:   {{ template "partials/mypartial.html" . }}
+
+`
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html", "Mypartial.")
+}
diff --git a/tpl/tplimpl/templates.go b/tpl/tplimpl/templates.go
index 4c3ad3be1..f3f98f622 100644
--- a/tpl/tplimpl/templates.go
+++ b/tpl/tplimpl/templates.go
@@ -99,7 +99,14 @@ func (t *templateNamespace) parseTemplate(ti *TemplInfo) error {
 					return err
 				}
 			}
+		}
 
+		// Issue #13599.
+		if ti.category == CategoryPartial && ti.Fi != nil && ti.Fi.Meta().PathInfo.Section() == "partials" {
+			aliasName := strings.TrimPrefix(name, "_")
+			if _, err := prototype.AddParseTree(aliasName, templ.(*htmltemplate.Template).Tree); err != nil {
+				return err
+			}
 		}
 	}
 
diff --git a/tpl/tplimpl/templatestore.go b/tpl/tplimpl/templatestore.go
index 8483b7df0..2e332e382 100644
--- a/tpl/tplimpl/templatestore.go
+++ b/tpl/tplimpl/templatestore.go
@@ -1,3 +1,18 @@
+// Copyright 2025 The Hugo Authors. All rights reserved.
+//
+// Portions Copyright The Go Authors.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package tplimpl
 
 import (
diff --git a/tpl/tplimpl/tplimpl_integration_test.go b/tpl/tplimpl/tplimpl_integration_test.go
index 8b80d5b60..b62898923 100644
--- a/tpl/tplimpl/tplimpl_integration_test.go
+++ b/tpl/tplimpl/tplimpl_integration_test.go
@@ -1,3 +1,18 @@
+// Copyright 2025 The Hugo Authors. All rights reserved.
+//
+// Portions Copyright The Go Authors.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package tplimpl_test
 
 import (