diff --git a/otherlibs/chrome-trace/src/chrome_trace.ml b/otherlibs/chrome-trace/src/chrome_trace.ml
index 21c30442b3b7..3ec19614f1e1 100644
--- a/otherlibs/chrome-trace/src/chrome_trace.ml
+++ b/otherlibs/chrome-trace/src/chrome_trace.ml
@@ -9,6 +9,7 @@ module Json = struct
     | `List of t list
     | `Bool of bool
     | `Assoc of (string * t) list
+    | `Null
     ]
 end
 
diff --git a/otherlibs/chrome-trace/src/chrome_trace.mli b/otherlibs/chrome-trace/src/chrome_trace.mli
index 31efcd14d6d5..e437cbab2549 100644
--- a/otherlibs/chrome-trace/src/chrome_trace.mli
+++ b/otherlibs/chrome-trace/src/chrome_trace.mli
@@ -21,6 +21,7 @@ module Json : sig
     | `List of t list
     | `Bool of bool
     | `Assoc of (string * t) list
+    | `Null
     ]
 end
 
diff --git a/src/dune_rules/odoc.ml b/src/dune_rules/odoc.ml
index 5916f315a0be..e166bab73f3b 100644
--- a/src/dune_rules/odoc.ml
+++ b/src/dune_rules/odoc.ml
@@ -139,6 +139,12 @@ module Output_format = struct
     match t with
     | Html -> Alias.doc ~dir
     | Json -> Alias.doc_json ~dir
+
+  let toplevel_index_path format ctx =
+    let base = Paths.toplevel_index ctx in
+    match format with
+    | Html -> base
+    | Json -> Path.Build.extend_basename base ~suffix:".json"
 end
 
 module Dep : sig
@@ -454,14 +460,44 @@ module Toplevel_index = struct
   </body>
 </html>|}
       Paths.odoc_support_dirname (html_list_items t)
+
+  let string_to_json s = `String s
+
+  let list_to_json ~f l = `List (List.map ~f l)
+
+  let option_to_json ~f = function
+    | None -> `Null
+    | Some x -> f x
+
+  let item_to_json { name; version; link } =
+    `Assoc
+      [ ("name", string_to_json name)
+      ; ("version", option_to_json ~f:string_to_json version)
+      ; ("link", string_to_json link)
+      ]
+
+  (** This format is public API. *)
+  let to_json items =
+    `Assoc [ ("packages", list_to_json items ~f:item_to_json) ]
+
+  let json t = Dune_stats.Json.to_string (to_json t)
+
+  let content (output : Output_format.t) t =
+    match output with
+    | Html -> html t
+    | Json -> json t
 end
 
-let setup_toplevel_index_rule sctx =
+let setup_toplevel_index_rule sctx output =
   let* packages = Only_packages.get () in
   let index = Toplevel_index.of_packages packages in
-  let html = Toplevel_index.html index in
+  let content = Toplevel_index.content output index in
   let ctx = Super_context.context sctx in
-  add_rule sctx (Action_builder.write_file (Paths.toplevel_index ctx) html)
+  let path = Output_format.toplevel_index_path output ctx in
+  add_rule sctx (Action_builder.write_file path content)
+
+let setup_toplevel_index_rules sctx =
+  Output_format.iter ~f:(setup_toplevel_index_rule sctx)
 
 let libs_of_pkg ctx ~pkg =
   let+ entries = Scope.DB.lib_entries_of_package ctx pkg in
@@ -517,10 +553,6 @@ let create_odoc ctx ~target odoc_file =
     in
     { odoc_file; odocl_file; html_file = file Html; json_file = file Json }
 
-let static_html ctx =
-  let open Paths in
-  [ odoc_support ctx; toplevel_index ctx ]
-
 let check_mlds_no_dupes ~pkg ~mlds =
   match
     List.rev_map mlds ~f:(fun mld ->
@@ -642,11 +674,12 @@ let out_file (output : Output_format.t) odoc =
 let out_files ctx (output : Output_format.t) odocs =
   let extra_files =
     match output with
-    | Html -> List.map ~f:Path.build (static_html ctx)
+    | Html -> [ Path.build (Paths.odoc_support ctx) ]
     | Json -> []
   in
-  List.rev_append extra_files
-    (List.map odocs ~f:(fun odoc -> Path.build (out_file output odoc)))
+  Path.build (Output_format.toplevel_index_path output ctx)
+  :: List.rev_append extra_files
+       (List.map odocs ~f:(fun odoc -> Path.build (out_file output odoc)))
 
 let setup_lib_html_rules_def =
   let module Input = struct
@@ -854,7 +887,7 @@ let gen_rules sctx ~dir rest =
       Path.Build.Map.singleton (Paths.odoc_support ctx) Loc.none
     in
     has_rules ~directory_targets
-      (setup_css_rule sctx >>> setup_toplevel_index_rule sctx)
+      (setup_css_rule sctx >>> setup_toplevel_index_rules sctx)
   | [ "_mlds"; pkg ] ->
     with_package pkg ~f:(fun pkg ->
         let* _mlds, rules = package_mlds sctx ~pkg in
diff --git a/src/dune_stats/dune_stats.ml b/src/dune_stats/dune_stats.ml
index 1f63a1cf7932..446d7ec52f23 100644
--- a/src/dune_stats/dune_stats.ml
+++ b/src/dune_stats/dune_stats.ml
@@ -66,6 +66,7 @@ module Json = struct
       Buffer.add_char buf '{';
       object_body_to_buf o buf;
       Buffer.add_char buf '}'
+    | `Null -> Buffer.add_string buf "null"
 
   and array_body_to_buf t buf =
     match t with
diff --git a/test/blackbox-tests/test-cases/odoc/doc-json.t b/test/blackbox-tests/test-cases/odoc/doc-json.t
index 9323a1a45e73..07b6fef465ec 100644
--- a/test/blackbox-tests/test-cases/odoc/doc-json.t
+++ b/test/blackbox-tests/test-cases/odoc/doc-json.t
@@ -22,15 +22,22 @@
 
   $ dune build @doc-json
   $ list_docs
+  _build/default/_doc/_html/index.html.json
   _build/default/_doc/_html/l/L/M/index.html.json
   _build/default/_doc/_html/l/L/index.html.json
   _build/default/_doc/_html/l/index.html.json
 
+The toplevel index is generated by dune itself:
+
+  $ cat _build/default/_doc/_html/index.html.json
+  {"packages":[{"name":"l","version":null,"link":"l/index.html"}]}
+
 @doc will continue generating doc as usual:
 
   $ dune build @doc
   $ list_docs
   _build/default/_doc/_html/index.html
+  _build/default/_doc/_html/index.html.json
   _build/default/_doc/_html/l/L/M/index.html
   _build/default/_doc/_html/l/L/M/index.html.json
   _build/default/_doc/_html/l/L/index.html