diff --git a/app/controllers/new_user/dossiers_controller.rb b/app/controllers/new_user/dossiers_controller.rb
index 060bae041..3a008190d 100644
--- a/app/controllers/new_user/dossiers_controller.rb
+++ b/app/controllers/new_user/dossiers_controller.rb
@@ -4,8 +4,8 @@ module NewUser
 
     helper_method :new_demarche_url
 
-    before_action :ensure_ownership!, except: [:index, :show, :demande, :modifier, :update, :recherche]
-    before_action :ensure_ownership_or_invitation!, only: [:show, :demande, :modifier, :update]
+    before_action :ensure_ownership!, except: [:index, :show, :demande, :messagerie, :modifier, :update, :recherche]
+    before_action :ensure_ownership_or_invitation!, only: [:show, :demande, :messagerie, :modifier, :update, :create_commentaire]
     before_action :ensure_dossier_can_be_updated, only: [:update_identite, :update]
     before_action :forbid_invite_submission!, only: [:update]
 
@@ -38,6 +38,11 @@ module NewUser
       @dossier = dossier
     end
 
+    def messagerie
+      @dossier = dossier
+      @commentaire = Commentaire.new
+    end
+
     def attestation
       send_data(dossier.attestation.pdf.read, filename: 'attestation.pdf', type: 'application/pdf')
     end
@@ -122,6 +127,18 @@ module NewUser
       @dossier = current_user.dossiers.includes(:procedure).find(params[:id])
     end
 
+    def create_commentaire
+      @commentaire = CommentaireService.create(current_user, dossier, commentaire_params)
+
+      if @commentaire.save
+        flash.notice = "Message envoyé"
+        redirect_to messagerie_dossier_path(dossier)
+      else
+        flash.now.alert = @commentaire.errors.full_messages
+        render :messagerie
+      end
+    end
+
     def ask_deletion
       dossier = current_user.dossiers.includes(:user, procedure: :administrateur).find(params[:id])
 
@@ -223,6 +240,10 @@ module NewUser
       params.require(:dossier).permit(:autorisation_donnees)
     end
 
+    def commentaire_params
+      params.require(:commentaire).permit(:body, :file)
+    end
+
     def passage_en_construction?
       dossier.brouillon? && !draft?
     end
diff --git a/app/views/new_user/dossiers/messagerie.html.haml b/app/views/new_user/dossiers/messagerie.html.haml
new file mode 100644
index 000000000..26d4fb8b6
--- /dev/null
+++ b/app/views/new_user/dossiers/messagerie.html.haml
@@ -0,0 +1,6 @@
+- content_for(:title, "Messagerie · Dossier nº #{@dossier.id} (#{@dossier.procedure.libelle})")
+
+#dossier-show
+  = render partial: 'new_user/dossiers/show/header', locals: { dossier: @dossier }
+
+  = render partial: "shared/dossiers/messagerie", locals: { dossier: @dossier, user_email: current_user.email, messagerie_seen_at: nil, new_commentaire: @commentaire, form_url: commentaire_dossier_path(@dossier) }
diff --git a/app/views/new_user/dossiers/show/_header.html.haml b/app/views/new_user/dossiers/show/_header.html.haml
index bc9d94dd7..52ae776c2 100644
--- a/app/views/new_user/dossiers/show/_header.html.haml
+++ b/app/views/new_user/dossiers/show/_header.html.haml
@@ -10,3 +10,4 @@
     %ul.tabs
       = active_tab_item('Résumé', dossier_path(dossier))
       = active_tab_item('Demande', demande_dossier_path(dossier))
+      = active_tab_item('Messagerie', messagerie_dossier_path(dossier))
diff --git a/config/routes.rb b/config/routes.rb
index 7f34ca76e..fc6b800e4 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -279,6 +279,8 @@ Rails.application.routes.draw do
         patch 'modifier', to: 'dossiers#update'
         get 'merci'
         get 'demande'
+        get 'messagerie'
+        post 'commentaire' => 'dossiers#create_commentaire'
         post 'ask_deletion'
         get 'attestation'
       end
diff --git a/spec/controllers/new_user/dossiers_controller_spec.rb b/spec/controllers/new_user/dossiers_controller_spec.rb
index 33cfd3566..36a7b2eea 100644
--- a/spec/controllers/new_user/dossiers_controller_spec.rb
+++ b/spec/controllers/new_user/dossiers_controller_spec.rb
@@ -523,6 +523,48 @@ describe NewUser::DossiersController, type: :controller do
     it { is_expected.to render_template(:demande) }
   end
 
+  describe "#create_commentaire" do
+    let(:dossier) { create(:dossier, :en_construction, user: user) }
+    let(:saved_commentaire) { dossier.commentaires.first }
+    let(:body) { "avant\napres" }
+    let(:file) { Rack::Test::UploadedFile.new("./spec/support/files/piece_justificative_0.pdf", 'application/pdf') }
+    let(:scan_result) { true }
+
+    subject {
+      post :create_commentaire, params: {
+        id: dossier.id,
+        commentaire: {
+          body: body,
+          file: file
+        }
+      }
+    }
+
+    before do
+      sign_in(user)
+      allow(ClamavService).to receive(:safe_file?).and_return(scan_result)
+    end
+
+    it "creates a commentaire" do
+      expect { subject }.to change(Commentaire, :count).by(1)
+
+      expect(response).to redirect_to(messagerie_dossier_path(dossier))
+      expect(flash.notice).to be_present
+    end
+
+    context "when the commentaire creation fails" do
+      let(:scan_result) { false }
+
+      it "renders the messagerie page with the invalid commentaire" do
+        expect { subject }.not_to change(Commentaire, :count)
+
+        expect(response).to render_template :messagerie
+        expect(flash.alert).to be_present
+        expect(assigns(:commentaire).body).to eq("<p>avant\n<br />apres</p>")
+      end
+    end
+  end
+
   describe '#ask_deletion' do
     before { sign_in(user) }
 
diff --git a/spec/features/new_user/dossier_details_spec.rb b/spec/features/new_user/dossier_details_spec.rb
index 4fc026e95..d6644e634 100644
--- a/spec/features/new_user/dossier_details_spec.rb
+++ b/spec/features/new_user/dossier_details_spec.rb
@@ -33,6 +33,27 @@ describe 'Dossier details:' do
     expect(page).to have_content('Nouveau texte')
   end
 
+  context 'with messages' do
+    let!(:commentaire) { create(:commentaire, dossier: dossier, email: 'instructeur@exemple.fr', body: 'Message envoyé à l’usager') }
+    let(:message_body) { 'Message envoyé à l’instructeur' }
+
+    scenario 'the user can send a message' do
+      visit_dossier dossier
+      click_on 'Messagerie'
+
+      expect(page).to have_current_path(messagerie_dossier_path(dossier))
+      expect(page).to have_content(commentaire.body)
+
+      fill_in 'commentaire_body', with: message_body
+      click_on 'Envoyer'
+
+      expect(page).to have_current_path(messagerie_dossier_path(dossier))
+      expect(page).to have_content('Message envoyé')
+      expect(page).to have_content(commentaire.body)
+      expect(page).to have_content(message_body)
+    end
+  end
+
   private
 
   def visit_dossier(dossier)