diff --git a/app/graphql/mutations/dossier_envoyer_message.rb b/app/graphql/mutations/dossier_envoyer_message.rb
new file mode 100644
index 000000000..601e448de
--- /dev/null
+++ b/app/graphql/mutations/dossier_envoyer_message.rb
@@ -0,0 +1,27 @@
+module Mutations
+  class DossierEnvoyerMessage < Mutations::BaseMutation
+    description "Envoyer un message à l'usager du dossier."
+
+    argument :dossier_id, ID, required: true, loads: Types::DossierType
+    argument :instructeur_id, ID, required: true, loads: Types::ProfileType
+    argument :body, String, required: true
+    argument :attachment, ID, required: false
+
+    field :message, Types::MessageType, null: true
+    field :errors, [Types::ValidationErrorType], null: true
+
+    def resolve(dossier:, instructeur:, body:, attachment: nil)
+      message = CommentaireService.build(instructeur, dossier, body: body, piece_jointe: attachment)
+
+      if message.save
+        { message: message }
+      else
+        { errors: message.errors.full_messages }
+      end
+    end
+
+    def authorized?(dossier:, instructeur:, body:)
+      instructeur.is_a?(Instructeur) && instructeur.dossiers.exists?(id: dossier.id)
+    end
+  end
+end
diff --git a/app/graphql/schema.graphql b/app/graphql/schema.graphql
index d43003eb3..b60fc03ae 100644
--- a/app/graphql/schema.graphql
+++ b/app/graphql/schema.graphql
@@ -352,6 +352,33 @@ type DossierEdge {
   node: Dossier
 }
 
+"""
+Autogenerated input type of DossierEnvoyerMessage
+"""
+input DossierEnvoyerMessageInput {
+  attachment: ID
+  body: String!
+
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+  dossierId: ID!
+  instructeurId: ID!
+}
+
+"""
+Autogenerated return type of DossierEnvoyerMessage
+"""
+type DossierEnvoyerMessagePayload {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+  errors: [ValidationError!]
+  message: Message
+}
+
 type DossierLinkChamp implements Champ {
   dossier: Dossier
   id: ID!
@@ -495,6 +522,11 @@ type Mutation {
   File information required to prepare a direct upload
   """
   createDirectUpload(input: CreateDirectUploadInput!): CreateDirectUploadPayload
+
+  """
+  Envoyer un message à l'usager du dossier.
+  """
+  dossierEnvoyerMessage(input: DossierEnvoyerMessageInput!): DossierEnvoyerMessagePayload
 }
 
 enum Order {
@@ -806,4 +838,14 @@ enum TypeDeChamp {
 """
 A valid URL, transported as a string
 """
-scalar URL
\ No newline at end of file
+scalar URL
+
+"""
+Éreur de validation
+"""
+type ValidationError {
+  """
+  A description of the error
+  """
+  message: String!
+}
\ No newline at end of file
diff --git a/app/graphql/types/mutation_type.rb b/app/graphql/types/mutation_type.rb
index bbdb5021e..92da07a80 100644
--- a/app/graphql/types/mutation_type.rb
+++ b/app/graphql/types/mutation_type.rb
@@ -1,5 +1,7 @@
 module Types
   class MutationType < Types::BaseObject
     field :create_direct_upload, mutation: Mutations::CreateDirectUpload
+
+    field :dossier_envoyer_message, mutation: Mutations::DossierEnvoyerMessage
   end
 end
diff --git a/app/graphql/types/validation_error_type.rb b/app/graphql/types/validation_error_type.rb
new file mode 100644
index 000000000..7df3a6fb4
--- /dev/null
+++ b/app/graphql/types/validation_error_type.rb
@@ -0,0 +1,11 @@
+module Types
+  class ValidationErrorType < Types::BaseObject
+    description "Éreur de validation"
+
+    field :message, String, "A description of the error", null: false
+
+    def message
+      object
+    end
+  end
+end
diff --git a/app/models/commentaire.rb b/app/models/commentaire.rb
index 5f7c4f11c..9a93319d4 100644
--- a/app/models/commentaire.rb
+++ b/app/models/commentaire.rb
@@ -10,7 +10,7 @@ class Commentaire < ApplicationRecord
 
   has_one_attached :piece_jointe
 
-  validates :body, presence: { message: "Votre message ne peut être vide" }
+  validates :body, presence: { message: "ne peut être vide" }
 
   default_scope { order(created_at: :asc) }
   scope :updated_since?, -> (date) { where('commentaires.updated_at > ?', date) }
diff --git a/config/locales/models/commentaire/fr.yml b/config/locales/models/commentaire/fr.yml
index 16b77c012..71ed7e25e 100644
--- a/config/locales/models/commentaire/fr.yml
+++ b/config/locales/models/commentaire/fr.yml
@@ -2,4 +2,5 @@ fr:
   activerecord:
     attributes:
       commentaire:
+        body: 'Votre message'
         file: fichier
diff --git a/spec/controllers/api/v2/graphql_controller_spec.rb b/spec/controllers/api/v2/graphql_controller_spec.rb
index 3d499d8e5..eb2721145 100644
--- a/spec/controllers/api/v2/graphql_controller_spec.rb
+++ b/spec/controllers/api/v2/graphql_controller_spec.rb
@@ -216,6 +216,81 @@ describe API::V2::GraphqlController do
     end
 
     context "mutations" do
+      describe 'dossierEnvoyerMessage' do
+        context 'success' do
+          let(:query) do
+            "mutation {
+              dossierEnvoyerMessage(input: {
+                dossierId: \"#{dossier.to_typed_id}\",
+                instructeurId: \"#{instructeur.to_typed_id}\",
+                body: \"Bonjour\"
+              }) {
+                message {
+                  body
+                }
+              }
+            }"
+          end
+
+          it "should post a message" do
+            expect(gql_errors).to eq(nil)
+
+            expect(gql_data).to eq(dossierEnvoyerMessage: {
+              message: {
+                body: "Bonjour"
+              }
+            })
+          end
+        end
+
+        context 'schema error' do
+          let(:query) do
+            "mutation {
+              dossierEnvoyerMessage(input: {
+                dossierId: \"#{dossier.to_typed_id}\",
+                instructeurId: \"#{instructeur.to_typed_id}\"
+              }) {
+                message {
+                  body
+                }
+              }
+            }"
+          end
+
+          it "should fail" do
+            expect(gql_data).to eq(nil)
+            expect(gql_errors).not_to eq(nil)
+          end
+        end
+
+        context 'validation error' do
+          let(:query) do
+            "mutation {
+              dossierEnvoyerMessage(input: {
+                dossierId: \"#{dossier.to_typed_id}\",
+                instructeurId: \"#{instructeur.to_typed_id}\",
+                body: \"\"
+              }) {
+                message {
+                  body
+                }
+                errors {
+                  message
+                }
+              }
+            }"
+          end
+
+          it "should fail" do
+            expect(gql_errors).to eq(nil)
+            expect(gql_data).to eq(dossierEnvoyerMessage: {
+              errors: [{ message: "Votre message ne peut être vide" }],
+              message: nil
+            })
+          end
+        end
+      end
+
       context 'createDirectUpload' do
         let(:query) do
           "mutation {
@@ -263,5 +338,26 @@ describe API::V2::GraphqlController do
         expect(gql_errors).not_to eq(nil)
       end
     end
+
+    context "mutation" do
+      let(:query) do
+        "mutation {
+          dossierEnvoyerMessage(input: {
+            dossierId: \"#{dossier.to_typed_id}\",
+            instructeurId: \"#{instructeur.to_typed_id}\",
+            body: \"Bonjour\"
+          }) {
+            message {
+              body
+            }
+          }
+        }"
+      end
+
+      it "should return error" do
+        expect(gql_data[:dossierEnvoyerMessage]).to eq(nil)
+        expect(gql_errors).not_to eq(nil)
+      end
+    end
   end
 end