diff --git a/app/validators/siret_format_validator.rb b/app/validators/siret_format_validator.rb
index c82b01f20..9bb9c3c7e 100644
--- a/app/validators/siret_format_validator.rb
+++ b/app/validators/siret_format_validator.rb
@@ -1,22 +1,35 @@
 class SiretFormatValidator < ActiveModel::EachValidator
-  def validate_each(record,attribute,value)
-    if !(value =~ /^\d{14}$/)
+  def validate_each(record, attribute, value)
+    if !format_is_valid(value)
       record.errors.add(attribute, :format)
     end
-    if value.nil? || (luhn_checksum(value) % 10 != 0)
+
+    if !luhn_passed(value)
       record.errors.add(attribute, :checksum)
     end
   end
 
   private
 
+  LA_POSTE_SIREN = '356000000'
+
+  def format_is_valid(value)
+    value.match?(/^\d{14}$/)
+  end
+
+  def luhn_passed(value)
+    # Do not enforce Luhn for La Poste SIRET numbers, the only exception to this rule
+    siret_is_attached_to_la_poste(value) || (luhn_checksum(value) % 10 == 0)
+  end
+
+  def siret_is_attached_to_la_poste(value)
+    value[0..8] == LA_POSTE_SIREN
+  end
+
   def luhn_checksum(value)
-    accum = 0
-    value.reverse.each_char.map(&:to_i).each_with_index do |digit, index|
+    value.reverse.each_char.map(&:to_i).map.with_index do |digit, index|
       t = index.even? ? digit : digit * 2
-      t = t - 9 if t >= 10
-      accum += t
-    end
-    accum
+      t < 10 ? t : t - 9
+    end.sum
   end
 end
diff --git a/spec/models/siret_spec.rb b/spec/models/siret_spec.rb
index c5d197660..579c2b916 100644
--- a/spec/models/siret_spec.rb
+++ b/spec/models/siret_spec.rb
@@ -1,26 +1,47 @@
 require 'spec_helper'
 
 describe Siret, type: :model do
-  let(:valid_siret) { '41816609600051' }
-  let(:invalid_siret) { '111111111' }
+  subject { Siret.new(siret: siret) }
 
   context 'with no siret provided' do
-    it { is_expected.to validate_presence_of(:siret) }
+    let(:siret) { '' }
+
+    it { is_expected.to be_invalid }
   end
 
-  context 'init with valid siret' do
-    it { is_expected.to allow_value(valid_siret).for(:siret) }
+  context 'with a siret that contains letters' do
+    let(:siret) { 'A1B1C6D9E0F0G1' }
+
+    it { is_expected.to be_invalid }
   end
 
-  context 'init with invalid siret' do
-    it { is_expected.not_to allow_value(invalid_siret).for(:siret) }
+  context 'with a siret that is too short' do
+    let(:siret) { '1234567890' }
+
+    it { is_expected.to be_invalid }
   end
 
-  context 'init with bullshit siret' do
-    it { is_expected.not_to allow_value('bullshit').for(:siret) }
+  context 'with a siret that is too long' do
+    let(:siret) { '12345678901234567890' }
+
+    it { is_expected.to be_invalid }
   end
 
-  context 'init with a siret that is too long' do
-    it { is_expected.not_to allow_value('9' * 15).for(:siret) }
+  context 'with a lunh-invalid siret' do
+    let(:siret) { '41816609600052' }
+
+    it { is_expected.to be_invalid }
+  end
+
+  context 'with a lunh-invalid La Poste siret' do
+    let(:siret) { '35600000018723' }
+
+    it { is_expected.to be_valid }
+  end
+
+  context 'with a valid siret' do
+    let(:siret) { '41816609600051' }
+
+    it { is_expected.to be_valid }
   end
 end