diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index f4c6b33a52924..3bc00635013eb 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -611,6 +611,15 @@ impl<'a> Parser<'a> {
                     appl,
                 );
             }
+
+            if ["def", "fun", "func", "function"].contains(&symbol.as_str()) {
+                err.span_suggestion_short(
+                    self.prev_token.span,
+                    &format!("write `fn` instead of `{symbol}` to declare a function"),
+                    "fn",
+                    appl,
+                );
+            }
         }
 
         // Add suggestion for a missing closing angle bracket if '>' is included in expected_tokens
diff --git a/src/test/ui/parser/fn-defined-using-def.rs b/src/test/ui/parser/fn-defined-using-def.rs
new file mode 100644
index 0000000000000..21da34c47c9fd
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-def.rs
@@ -0,0 +1,10 @@
+// Check what happens when `def` is used to define a function, instead of `fn`
+// edition:2021
+
+#![allow(dead_code)]
+
+def foo() {}
+//~^ ERROR expected one of `!` or `::`, found `foo`
+//~^^ HELP write `fn` instead of `def` to declare a function
+
+fn main() {}
diff --git a/src/test/ui/parser/fn-defined-using-def.stderr b/src/test/ui/parser/fn-defined-using-def.stderr
new file mode 100644
index 0000000000000..f34329012a02f
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-def.stderr
@@ -0,0 +1,10 @@
+error: expected one of `!` or `::`, found `foo`
+  --> $DIR/fn-defined-using-def.rs:6:5
+   |
+LL | def foo() {}
+   | --- ^^^ expected one of `!` or `::`
+   | |
+   | help: write `fn` instead of `def` to declare a function
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/parser/fn-defined-using-fun.rs b/src/test/ui/parser/fn-defined-using-fun.rs
new file mode 100644
index 0000000000000..4f74605043e19
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-fun.rs
@@ -0,0 +1,10 @@
+// Check what happens when `fun` is used to define a function, instead of `fn`
+// edition:2021
+
+#![allow(dead_code)]
+
+fun foo() {}
+//~^ ERROR expected one of `!` or `::`, found `foo`
+//~^^ HELP write `fn` instead of `fun` to declare a function
+
+fn main() {}
diff --git a/src/test/ui/parser/fn-defined-using-fun.stderr b/src/test/ui/parser/fn-defined-using-fun.stderr
new file mode 100644
index 0000000000000..2f6cfff350c90
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-fun.stderr
@@ -0,0 +1,10 @@
+error: expected one of `!` or `::`, found `foo`
+  --> $DIR/fn-defined-using-fun.rs:6:5
+   |
+LL | fun foo() {}
+   | --- ^^^ expected one of `!` or `::`
+   | |
+   | help: write `fn` instead of `fun` to declare a function
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/parser/fn-defined-using-func.rs b/src/test/ui/parser/fn-defined-using-func.rs
new file mode 100644
index 0000000000000..2dce96fdce078
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-func.rs
@@ -0,0 +1,10 @@
+// Check what happens when `func` is used to define a function, instead of `fn`
+// edition:2021
+
+#![allow(dead_code)]
+
+func foo() {}
+//~^ ERROR expected one of `!` or `::`, found `foo`
+//~^^ HELP write `fn` instead of `func` to declare a function
+
+fn main() {}
diff --git a/src/test/ui/parser/fn-defined-using-func.stderr b/src/test/ui/parser/fn-defined-using-func.stderr
new file mode 100644
index 0000000000000..355741e8949a9
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-func.stderr
@@ -0,0 +1,10 @@
+error: expected one of `!` or `::`, found `foo`
+  --> $DIR/fn-defined-using-func.rs:6:6
+   |
+LL | func foo() {}
+   | ---- ^^^ expected one of `!` or `::`
+   | |
+   | help: write `fn` instead of `func` to declare a function
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/parser/fn-defined-using-function.rs b/src/test/ui/parser/fn-defined-using-function.rs
new file mode 100644
index 0000000000000..fd8782728e2b9
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-function.rs
@@ -0,0 +1,10 @@
+// Check what happens when `function` is used to define a function, instead of `fn`
+// edition:2021
+
+#![allow(dead_code)]
+
+function foo() {}
+//~^ ERROR expected one of `!` or `::`, found `foo`
+//~^^ HELP write `fn` instead of `function` to declare a function
+
+fn main() {}
diff --git a/src/test/ui/parser/fn-defined-using-function.stderr b/src/test/ui/parser/fn-defined-using-function.stderr
new file mode 100644
index 0000000000000..43c33a2cdd7e8
--- /dev/null
+++ b/src/test/ui/parser/fn-defined-using-function.stderr
@@ -0,0 +1,10 @@
+error: expected one of `!` or `::`, found `foo`
+  --> $DIR/fn-defined-using-function.rs:6:10
+   |
+LL | function foo() {}
+   | -------- ^^^ expected one of `!` or `::`
+   | |
+   | help: write `fn` instead of `function` to declare a function
+
+error: aborting due to previous error
+