diff --git a/js/src/graphql/event.ts b/js/src/graphql/event.ts
index c0e1e904..87e54ecb 100644
--- a/js/src/graphql/event.ts
+++ b/js/src/graphql/event.ts
@@ -295,7 +295,7 @@ export const JOIN_EVENT = gql`
     $email: String
     $message: String
     $locale: String
-    $timezone: String
+    $timezone: Timezone
   ) {
     joinEvent(
       eventId: $eventId
diff --git a/js/src/graphql/user.ts b/js/src/graphql/user.ts
index e902488f..33ef1bff 100644
--- a/js/src/graphql/user.ts
+++ b/js/src/graphql/user.ts
@@ -151,7 +151,7 @@ export const LOGGED_USER_TIMEZONE = gql`
 
 export const SET_USER_SETTINGS = gql`
   mutation SetUserSettings(
-    $timezone: String
+    $timezone: Timezone
     $notificationOnDay: Boolean
     $notificationEachWeek: Boolean
     $notificationBeforeEvent: Boolean
diff --git a/lib/graphql/schema.ex b/lib/graphql/schema.ex
index b74385db..b635bc72 100644
--- a/lib/graphql/schema.ex
+++ b/lib/graphql/schema.ex
@@ -31,6 +31,7 @@ defmodule Mobilizon.GraphQL.Schema do
   import_types(Absinthe.Plug.Types)
   import_types(Custom.UUID)
   import_types(Custom.Point)
+  import_types(Custom.Timezone)
 
   import_types(Schema.ActivityType)
   import_types(Schema.UserType)
diff --git a/lib/graphql/schema/address.ex b/lib/graphql/schema/address.ex
index 0ca407f1..f1ec42a9 100644
--- a/lib/graphql/schema/address.ex
+++ b/lib/graphql/schema/address.ex
@@ -22,7 +22,7 @@ defmodule Mobilizon.GraphQL.Schema.AddressType do
     field(:url, :string, description: "The address's URL")
     field(:id, :id, description: "The address's ID")
     field(:origin_id, :string, description: "The address's original ID from the provider")
-    field(:timezone, :string, description: "The (estimated) timezone of the location")
+    field(:timezone, :timezone, description: "The (estimated) timezone of the location")
     field(:picture_info, :picture_info, description: "A picture associated with the address")
   end
 
@@ -75,7 +75,7 @@ defmodule Mobilizon.GraphQL.Schema.AddressType do
     field(:url, :string, description: "The address's URL")
     field(:id, :id, description: "The address's ID")
     field(:origin_id, :string, description: "The address's original ID from the provider")
-    field(:timezone, :string, description: "The (estimated) timezone of the location")
+    field(:timezone, :timezone, description: "The (estimated) timezone of the location")
   end
 
   @desc """
diff --git a/lib/graphql/schema/config.ex b/lib/graphql/schema/config.ex
index 19c2fbec..cc42f1b1 100644
--- a/lib/graphql/schema/config.ex
+++ b/lib/graphql/schema/config.ex
@@ -43,7 +43,7 @@ defmodule Mobilizon.GraphQL.Schema.ConfigType do
 
     field(:upload_limits, :upload_limits, description: "The configuration for upload limits")
 
-    field(:timezones, list_of(:string), description: "The instance's available timezones")
+    field(:timezones, list_of(:timezone), description: "The instance's available timezones")
     field(:features, :features, description: "The instance's features")
     field(:restrictions, :restrictions, description: "The instance's restrictions")
     field(:version, :string, description: "The instance's version")
diff --git a/lib/graphql/schema/custom/timezone.ex b/lib/graphql/schema/custom/timezone.ex
new file mode 100644
index 00000000..cd20efc0
--- /dev/null
+++ b/lib/graphql/schema/custom/timezone.ex
@@ -0,0 +1,35 @@
+defmodule Mobilizon.GraphQL.Schema.Custom.Timezone do
+  @moduledoc """
+  The timezone scalar type allows timezone ID strings to be passed in and out.
+  """
+  use Absinthe.Schema.Notation
+  import Mobilizon.Web.Gettext, only: [dgettext: 3]
+
+  scalar :timezone, name: "Timezone" do
+    description("""
+    The `Timezone` scalar type represents a timezone identifier,
+    as registered in the IANA Time Zone Database.
+    """)
+
+    serialize(&encode/1)
+    parse(&decode/1)
+  end
+
+  @spec decode(Absinthe.Blueprint.Input.String.t()) :: {:ok, term} | :error
+  @spec decode(Absinthe.Blueprint.Input.Null.t()) :: {:ok, nil}
+  defp decode(%Absinthe.Blueprint.Input.String{value: value}) do
+    if Tzdata.zone_exists?(value),
+      do: {:ok, value},
+      else: {:error, dgettext("errors", "Timezone ID %{timezone} is invalid", timezone: value)}
+  end
+
+  defp decode(%Absinthe.Blueprint.Input.Null{}) do
+    {:ok, nil}
+  end
+
+  defp decode(_) do
+    :error
+  end
+
+  defp encode(value), do: value
+end
diff --git a/lib/graphql/schema/event.ex b/lib/graphql/schema/event.ex
index a44294ef..544f565d 100644
--- a/lib/graphql/schema/event.ex
+++ b/lib/graphql/schema/event.ex
@@ -250,7 +250,7 @@ defmodule Mobilizon.GraphQL.Schema.EventType do
     field(:show_start_time, :boolean, description: "Show event start time")
     field(:show_end_time, :boolean, description: "Show event end time")
 
-    field(:timezone, :string, description: "The event's timezone")
+    field(:timezone, :timezone, description: "The event's timezone")
 
     field(:hide_organizer_when_group_event, :boolean,
       description:
@@ -303,7 +303,7 @@ defmodule Mobilizon.GraphQL.Schema.EventType do
     field(:show_start_time, :boolean, description: "Show event start time")
     field(:show_end_time, :boolean, description: "Show event end time")
 
-    field(:timezone, :string, description: "The event's timezone")
+    field(:timezone, :timezone, description: "The event's timezone")
 
     field(:hide_organizer_when_group_event, :boolean,
       description:
diff --git a/lib/graphql/schema/events/participant.ex b/lib/graphql/schema/events/participant.ex
index 6e61af84..457e5b14 100644
--- a/lib/graphql/schema/events/participant.ex
+++ b/lib/graphql/schema/events/participant.ex
@@ -96,7 +96,7 @@ defmodule Mobilizon.GraphQL.Schema.Events.ParticipantType do
       arg(:email, :string, description: "The anonymous participant's email")
       arg(:message, :string, description: "The anonymous participant's message")
       arg(:locale, :string, description: "The anonymous participant's locale")
-      arg(:timezone, :string, description: "The anonymous participant's timezone")
+      arg(:timezone, :timezone, description: "The anonymous participant's timezone")
       middleware(Rajska.QueryAuthorization, permit: :all, rule: :"write:participation")
       resolve(&Participant.actor_join_event/3)
     end
diff --git a/lib/graphql/schema/user.ex b/lib/graphql/schema/user.ex
index 9fc64053..5588a414 100644
--- a/lib/graphql/schema/user.ex
+++ b/lib/graphql/schema/user.ex
@@ -220,7 +220,7 @@ defmodule Mobilizon.GraphQL.Schema.UserType do
   """
   object :user_settings do
     meta(:authorize, :user)
-    field(:timezone, :string, description: "The timezone for this user")
+    field(:timezone, :timezone, description: "The timezone for this user")
 
     field(:notification_on_day, :boolean,
       description: "Whether this user will receive an email at the start of the day of an event."
@@ -450,7 +450,7 @@ defmodule Mobilizon.GraphQL.Schema.UserType do
 
     @desc "Set user settings"
     field :set_user_settings, :user_settings do
-      arg(:timezone, :string, description: "The timezone for this user")
+      arg(:timezone, :timezone, description: "The timezone for this user")
 
       arg(:notification_on_day, :boolean,
         description: