diff --git a/lib/ecto/adapters/sql/sandbox.ex b/lib/ecto/adapters/sql/sandbox.ex
index f40deb42..78b5b3c1 100644
--- a/lib/ecto/adapters/sql/sandbox.ex
+++ b/lib/ecto/adapters/sql/sandbox.ex
@@ -436,6 +436,7 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   The remaining options are passed to `checkout/2`.
   """
   @doc since: "3.4.4"
+  @spec start_owner!(Ecto.Repo.t() | pid(), keyword()) :: pid()
   def start_owner!(repo, opts \\ []) do
     parent = self()
 
@@ -489,7 +490,15 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   connections are checked in. Therefore, it is recommend to set those
   modes before your test suite starts, as otherwise you will check in
   connections being used in any other test running concurrently.
+
+  If successful, returns `:ok` (this is always successful for `:auto`
+  and `:manual` modes). It may return `:not_owner` or `:not_found`
+  when setting `{:shared, pid}` and the given `pid` does not own any
+  connection for the repo. May return `:already_shared` if another
+  process set the ownership mode to `{:shared, _}` and is still alive.
   """
+  @spec mode(Ecto.Repo.t() | pid(), :auto | :manual | {:shared, pid()}) ::
+          :ok | :already_shared | :now_owner | :not_found
   def mode(repo, mode)
       when (is_atom(repo) or is_pid(repo)) and mode in [:auto, :manual]
       when (is_atom(repo) or is_pid(repo)) and elem(mode, 0) == :shared and is_pid(elem(mode, 1)) do
@@ -504,6 +513,9 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   until it calls `checkin/2` or until it crashes in which case
   the connection will be automatically reclaimed by the pool.
 
+  If successful, returns `:ok`. If the caller already has a
+  connection, it returns `{:already, :owner | :allowed}`.
+
   ## Options
 
     * `:sandbox` - when true the connection is wrapped in
@@ -519,6 +531,7 @@ defmodule Ecto.Adapters.SQL.Sandbox do
       be bumped whenever necessary.
 
   """
+  @spec checkout(Ecto.Repo.t() | pid(), keyword()) :: :ok | {:already, :owner | :allowed}
   def checkout(repo, opts \\ []) when is_atom(repo) or is_pid(repo) do
     %{pid: pool, opts: pool_opts} = lookup_meta!(repo)
 
@@ -564,6 +577,7 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   @doc """
   Checks in the connection back into the sandbox pool.
   """
+  @spec checkin(Ecto.Repo.t() | pid()) :: :ok | :not_owner | :not_found
   def checkin(repo, _opts \\ []) when is_atom(repo) or is_pid(repo) do
     %{pid: pool, opts: opts} = lookup_meta!(repo)
     DBConnection.Ownership.ownership_checkin(pool, opts)
@@ -573,7 +587,13 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   Allows the `allow` process to use the same connection as `parent`.
 
   `allow` may be a PID or a locally registered name.
+
+  If the allowance is successful, this function returns `:ok`. If `allow` is already an
+  owner or already allowed, it returns `{:already, :owner | :allowed}`. If `parent` has not
+  checked out a connection from the repo, it returns `:not_found`.
   """
+  @spec allow(Ecto.Repo.t() | pid(), pid(), term()) ::
+          :ok | {:already, :owner | :allowed} | :not_found
   def allow(repo, parent, allow, _opts \\ []) when is_atom(repo) or is_pid(repo) do
     case GenServer.whereis(allow) do
       pid when is_pid(pid) ->
@@ -591,6 +611,7 @@ defmodule Ecto.Adapters.SQL.Sandbox do
   @doc """
   Runs a function outside of the sandbox.
   """
+  @spec unboxed_run(Ecto.Repo.t() | pid(), (-> result)) :: result when result: var
   def unboxed_run(repo, fun) when is_atom(repo) or is_pid(repo) do
     checkin(repo)
     checkout(repo, sandbox: false)