diff --git a/app/controllers/instructeurs/avis_controller.rb b/app/controllers/instructeurs/avis_controller.rb
index 0b280cd8e..8b2c0487c 100644
--- a/app/controllers/instructeurs/avis_controller.rb
+++ b/app/controllers/instructeurs/avis_controller.rb
@@ -6,7 +6,7 @@ module Instructeurs
     before_action :check_if_avis_revoked, only: [:show]
     before_action :redirect_if_no_sign_up_needed, only: [:sign_up]
     before_action :check_avis_exists_and_email_belongs_to_avis, only: [:sign_up, :create_instructeur]
-    before_action :set_avis_and_dossier, only: [:show, :instruction, :update]
+    before_action :set_avis_and_dossier, only: [:show, :instruction, :messagerie, :create_commentaire, :update]
 
     A_DONNER_STATUS = 'a-donner'
     DONNES_STATUS   = 'donnes'
@@ -53,6 +53,23 @@ module Instructeurs
       end
     end
 
+    def messagerie
+      @commentaire = Commentaire.new
+    end
+
+    def create_commentaire
+      @commentaire = CommentaireService.build(current_instructeur, avis.dossier, commentaire_params)
+
+      if @commentaire.save
+        @commentaire.dossier.update!(last_commentaire_updated_at: Time.zone.now)
+        flash.notice = "Message envoyé"
+        redirect_to messagerie_instructeur_avis_path(avis.procedure, avis)
+      else
+        flash.alert = @commentaire.errors.full_messages
+        render :messagerie
+      end
+    end
+
     def create_avis
       @procedure = Procedure.find(params[:procedure_id])
       if !feature_enabled_for?(:expert_not_allowed_to_invite, @procedure)
diff --git a/app/views/instructeurs/avis/_header.html.haml b/app/views/instructeurs/avis/_header.html.haml
index 1c01bf985..0d2ab24e6 100644
--- a/app/views/instructeurs/avis/_header.html.haml
+++ b/app/views/instructeurs/avis/_header.html.haml
@@ -8,3 +8,4 @@
     %ul.tabs
       = dynamic_tab_item('Demande', instructeur_avis_path(avis.procedure, avis))
       = dynamic_tab_item('Avis', instruction_instructeur_avis_path(avis.procedure, avis), notification: avis.answer.blank?)
+      = dynamic_tab_item('Messagerie', messagerie_instructeur_avis_path(avis.procedure, avis))
diff --git a/app/views/instructeurs/avis/messagerie.html.haml b/app/views/instructeurs/avis/messagerie.html.haml
new file mode 100644
index 000000000..46e64e69a
--- /dev/null
+++ b/app/views/instructeurs/avis/messagerie.html.haml
@@ -0,0 +1,5 @@
+- content_for(:title, "Messagerie · Dossier nº #{@dossier.id} (#{@dossier.owner_name})")
+
+= render partial: 'header', locals: { avis: @avis, dossier: @dossier }
+
+= render partial: "shared/dossiers/messagerie", locals: { dossier: @dossier, connected_user: current_instructeur, messagerie_seen_at: nil, new_commentaire: @commentaire, form_url: commentaire_instructeur_avis_path(@avis) }
diff --git a/config/routes.rb b/config/routes.rb
index 405a06824..b207161a6 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -300,6 +300,7 @@ Rails.application.routes.draw do
           get '', action: 'procedure', on: :collection, as: :procedure
           member do
             get 'instruction'
+            get 'messagerie'
             post 'commentaire' => 'avis#create_commentaire'
             post 'avis' => 'avis#create_avis'
             patch 'revoquer'
diff --git a/spec/controllers/instructeurs/avis_controller_spec.rb b/spec/controllers/instructeurs/avis_controller_spec.rb
index 6b6ffe4cc..720578b19 100644
--- a/spec/controllers/instructeurs/avis_controller_spec.rb
+++ b/spec/controllers/instructeurs/avis_controller_spec.rb
@@ -65,6 +65,14 @@ describe Instructeurs::AvisController, type: :controller do
       it { expect(assigns(:dossier)).to eq(dossier) }
     end
 
+    describe '#messagerie' do
+      before { get :messagerie, params: { id: avis_without_answer.id, procedure_id: procedure.id } }
+
+      it { expect(response).to have_http_status(:success) }
+      it { expect(assigns(:avis)).to eq(avis_without_answer) }
+      it { expect(assigns(:dossier)).to eq(dossier) }
+    end
+
     describe '#bilans_bdf' do
       before { get :bilans_bdf, params: { id: avis_without_answer.id, procedure_id: procedure.id } }
 
@@ -110,6 +118,40 @@ describe Instructeurs::AvisController, type: :controller do
       end
     end
 
+    describe '#create_commentaire' do
+      let(:file) { nil }
+      let(:scan_result) { true }
+      let(:now) { Time.zone.parse("14/07/1789") }
+
+      subject { post :create_commentaire, params: { id: avis_without_answer.id, procedure_id: procedure.id, commentaire: { body: 'commentaire body', piece_jointe: file } } }
+
+      before do
+        allow(ClamavService).to receive(:safe_file?).and_return(scan_result)
+        Timecop.freeze(now)
+      end
+
+      after { Timecop.return }
+
+      it do
+        subject
+
+        expect(response).to redirect_to(messagerie_instructeur_avis_path(avis_without_answer.procedure, avis_without_answer))
+        expect(dossier.commentaires.map(&:body)).to match(['commentaire body'])
+        expect(dossier.reload.last_commentaire_updated_at).to eq(now)
+      end
+
+      context "with a file" do
+        let(:file) { fixture_file_upload('spec/fixtures/files/piece_justificative_0.pdf', 'application/pdf') }
+
+        it do
+          subject
+          expect(Commentaire.last.piece_jointe.filename).to eq("piece_justificative_0.pdf")
+        end
+
+        it { expect { subject }.to change(Commentaire, :count).by(1) }
+      end
+    end
+
     describe '#expert_cannot_invite_another_expert' do
       let!(:previous_avis) { Avis.create(dossier: dossier, claimant: claimant, instructeur: instructeur, confidentiel: previous_avis_confidentiel) }
       let(:previous_avis_confidentiel) { false }