diff --git a/macros/src/types/enum.rs b/macros/src/types/enum.rs
index 4c73b756a..0cfcfd9b2 100644
--- a/macros/src/types/enum.rs
+++ b/macros/src/types/enum.rs
@@ -50,7 +50,11 @@ pub(crate) fn r#enum_def(s: &ItemEnum) -> syn::Result<DerivedTS> {
     Ok(DerivedTS {
         inline: quote!([#(#formatted_variants),*].join(" | ")),
         decl: quote!(format!("type {}{} = {};", #name, #generic_args, Self::inline())),
-        inline_flattened: None,
+        inline_flattened: Some(
+            quote!(
+                format!("({})", [#(#formatted_variants),*].join(" | "))
+            )
+        ),
         dependencies,
         name,
         export: enum_attr.export,
@@ -130,7 +134,15 @@ fn format_variant(
                     "{{ \"{}\": \"{}\", {} }}",
                     #tag,
                     #name,
-                    #inline_flattened
+                    // At this point inline_flattened looks like
+                    // { /* ...data */ }
+                    // 
+                    // To be flattened, an internally tagged enum must not be
+                    // surrounded by braces, otherwise each variant will look like
+                    // { "tag": "name", { /* ...data */ } }
+                    // when we want it to look like
+                    // { "tag": "name", /* ...data */ }
+                    #inline_flattened.trim_matches(&['{', '}', ' '])
                 )
             },
             None => match &variant.fields {
diff --git a/macros/src/types/named.rs b/macros/src/types/named.rs
index 1a3249b42..3ec97d014 100644
--- a/macros/src/types/named.rs
+++ b/macros/src/types/named.rs
@@ -17,6 +17,7 @@ pub(crate) fn named(
     generics: &Generics,
 ) -> Result<DerivedTS> {
     let mut formatted_fields = Vec::new();
+    let mut flattened_fields = Vec::new();
     let mut dependencies = Dependencies::default();
     if let Some(tag) = &attr.tag {
         let formatted = format!("{}: \"{}\",", tag, name);
@@ -28,6 +29,7 @@ pub(crate) fn named(
     for field in &fields.named {
         format_field(
             &mut formatted_fields,
+            &mut flattened_fields,
             &mut dependencies,
             field,
             &attr.rename_all,
@@ -36,17 +38,21 @@ pub(crate) fn named(
     }
 
     let fields = quote!(<[String]>::join(&[#(#formatted_fields),*], " "));
+    let flattened = quote!(<[String]>::join(&[#(#flattened_fields),*], " & "));
     let generic_args = format_generics(&mut dependencies, generics);
 
+    let inline = match (formatted_fields.len(), flattened_fields.len()) {
+        (0, 0) => quote!("{  }".to_owned()),
+        (_, 0) => quote!(format!("{{ {} }}", #fields)),
+        (0, 1) => quote!(#flattened.trim_matches(|c| c == '(' || c == ')').to_owned()),
+        (0, _) => quote!(#flattened),
+        (_, _) => quote!(format!("{{ {} }} & {}", #fields, #flattened)),
+    };
+
     Ok(DerivedTS {
-        inline: quote! {
-            format!(
-                "{{ {} }}",
-                #fields,
-            )
-        },
+        inline: quote!(#inline.replace(" } & { ", " ")),
         decl: quote!(format!("type {}{} = {}", #name, #generic_args, Self::inline())),
-        inline_flattened: Some(fields),
+        inline_flattened: Some(quote!(format!("{{ {} }}", #fields))),
         name: name.to_owned(),
         dependencies,
         export: attr.export,
@@ -55,8 +61,18 @@ pub(crate) fn named(
 }
 
 // build an expresion which expands to a string, representing a single field of a struct.
+//
+// formatted_fields will contain all the fields that do not contain the flatten
+// attribute, in the format
+// key: type,
+//
+// flattened_fields will contain all the fields that contain the flatten attribute
+// in their respective formats, which for a named struct is the same as formatted_fields,
+// but for enums is
+// ({ /* variant data */ } | { /* variant data */ })
 fn format_field(
     formatted_fields: &mut Vec<TokenStream>,
+    flattened_fields: &mut Vec<TokenStream>,
     dependencies: &mut Dependencies,
     field: &Field,
     rename_all: &Option<Inflection>,
@@ -88,7 +104,7 @@ fn format_field(
             _ => {}
         }
 
-        formatted_fields.push(quote!(<#ty as ts_rs::TS>::inline_flattened()));
+        flattened_fields.push(quote!(<#ty as ts_rs::TS>::inline_flattened()));
         dependencies.append_from(ty);
         return Ok(());
     }
diff --git a/ts-rs/tests/enum_flattening.rs b/ts-rs/tests/enum_flattening.rs
new file mode 100644
index 000000000..c26ef059a
--- /dev/null
+++ b/ts-rs/tests/enum_flattening.rs
@@ -0,0 +1,108 @@
+#[cfg(feature = "serde-compat")]
+use serde::Serialize;
+use ts_rs::TS;
+
+#[test]
+fn externally_tagged() {
+    #[allow(dead_code)]
+    #[cfg_attr(feature = "serde-compat", derive(Serialize, TS))]
+    #[cfg_attr(not(feature = "serde-compat"), derive(TS))]
+    struct Foo {
+        qux: i32,
+        #[cfg_attr(feature = "serde-compat", serde(flatten))]
+        #[cfg_attr(not(feature = "serde-compat"), ts(flatten))]
+        baz: Bar,
+        biz: Option<String>,
+    }
+
+    #[cfg_attr(feature = "serde-compat", derive(Serialize, TS))]
+    #[cfg_attr(not(feature = "serde-compat"), derive(TS))]
+    #[allow(dead_code)]
+    enum Bar {
+        Baz { a: i32, a2: String },
+        Biz { b: bool },
+        Buz { c: String, d: Option<i32> },
+    }
+
+    assert_eq!(
+        Foo::inline(),
+        r#"{ qux: number, biz: string | null, } & ({ "Baz": { a: number, a2: string, } } | { "Biz": { b: boolean, } } | { "Buz": { c: string, d: number | null, } })"#
+    )
+}
+
+#[test]
+#[cfg(feature = "serde-compat")]
+fn adjacently_tagged() {
+    #[derive(Serialize, TS)]
+    struct Foo {
+        one: i32,
+        #[serde(flatten)]
+        baz: Bar,
+        qux: Option<String>,
+    }
+
+    #[derive(Serialize, TS)]
+    #[allow(dead_code)]
+    #[serde(tag = "type", content = "stuff")]
+    enum Bar {
+        Baz { a: i32, a2: String },
+        Biz { b: bool },
+        Buz { c: String, d: Option<i32> },
+    }
+
+    assert_eq!(
+        Foo::inline(),
+        r#"{ one: number, qux: string | null, } & ({ "type": "Baz", "stuff": { a: number, a2: string, } } | { "type": "Biz", "stuff": { b: boolean, } } | { "type": "Buz", "stuff": { c: string, d: number | null, } })"#
+    )
+}
+
+#[test]
+#[cfg(feature = "serde-compat")]
+fn internally_tagged() {
+    #[derive(Serialize, TS)]
+    struct Foo {
+        qux: Option<String>,
+
+        #[serde(flatten)]
+        baz: Bar,
+    }
+
+    #[derive(Serialize, TS)]
+    #[allow(dead_code)]
+    #[serde(tag = "type")]
+    enum Bar {
+        Baz { a: i32, a2: String },
+        Biz { b: bool },
+        Buz { c: String, d: Option<i32> },
+    }
+
+    assert_eq!(
+        Foo::inline(),
+        r#"{ qux: string | null, } & ({ "type": "Baz", a: number, a2: string, } | { "type": "Biz", b: boolean, } | { "type": "Buz", c: string, d: number | null, })"#
+    )
+}
+
+#[test]
+#[cfg(feature = "serde-compat")]
+fn untagged() {
+    #[derive(Serialize, TS)]
+    struct Foo {
+        #[serde(flatten)]
+        baz: Bar,
+    }
+
+    #[derive(Serialize, TS)]
+    #[allow(dead_code)]
+    #[serde(untagged)]
+    enum Bar {
+        Baz { a: i32, a2: String },
+        Biz { b: bool },
+        Buz { c: String },
+    }
+
+    assert_eq!(
+        Foo::inline(),
+        r#"{ a: number, a2: string, } | { b: boolean, } | { c: string, }"#
+    )
+}
+
diff --git a/ts-rs/tests/flatten.rs b/ts-rs/tests/flatten.rs
index 837c9b65b..56212498f 100644
--- a/ts-rs/tests/flatten.rs
+++ b/ts-rs/tests/flatten.rs
@@ -26,6 +26,6 @@ struct C {
 fn test_def() {
     assert_eq!(
         C::inline(),
-        "{ b: { a: number, b: number, c: number, }, d: number, }"
+        "{ b: { c: number, a: number, b: number, }, d: number, }"
     );
 }