diff --git a/app/assets/javascripts/discourse/app/templates/modal/request-group-membership-form.hbs b/app/assets/javascripts/discourse/app/templates/modal/request-group-membership-form.hbs
index dbd26377b8f..3d171b6f678 100644
--- a/app/assets/javascripts/discourse/app/templates/modal/request-group-membership-form.hbs
+++ b/app/assets/javascripts/discourse/app/templates/modal/request-group-membership-form.hbs
@@ -5,7 +5,7 @@
         {{i18n "groups.membership_request.reason"}}
       </label>
 
-      <ExpandingTextArea @value={{this.reason}} />
+      <ExpandingTextArea @value={{this.reason}} @maxlength="280" />
     </div>
   </DModalBody>
 
diff --git a/app/models/group_request.rb b/app/models/group_request.rb
index 706fcceab97..c923622aac3 100644
--- a/app/models/group_request.rb
+++ b/app/models/group_request.rb
@@ -1,8 +1,12 @@
 # frozen_string_literal: true
 
 class GroupRequest < ActiveRecord::Base
+  REASON_CHARACTER_LIMIT = 280
+
   belongs_to :group
   belongs_to :user
+
+  validates :reason, length: { maximum: REASON_CHARACTER_LIMIT }
 end
 
 # == Schema Information
diff --git a/spec/models/group_request_spec.rb b/spec/models/group_request_spec.rb
new file mode 100644
index 00000000000..3dd9a660a31
--- /dev/null
+++ b/spec/models/group_request_spec.rb
@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+RSpec.describe GroupRequest do
+  it { is_expected.to belong_to :user }
+  it { is_expected.to belong_to :group }
+
+  it do
+    is_expected.to validate_length_of(:reason).is_at_most(described_class::REASON_CHARACTER_LIMIT)
+  end
+end
diff --git a/spec/requests/groups_controller_spec.rb b/spec/requests/groups_controller_spec.rb
index 6b432c1b2a7..87c7ad0628e 100644
--- a/spec/requests/groups_controller_spec.rb
+++ b/spec/requests/groups_controller_spec.rb
@@ -2198,6 +2198,20 @@ RSpec.describe GroupsController do
       expect(response.status).to eq(409)
     end
 
+    it "limits the character count of the reason" do
+      sign_in(user)
+
+      post "/groups/#{group.name}/request_membership.json",
+           params: {
+             reason: "x" * (GroupRequest::REASON_CHARACTER_LIMIT + 1),
+           }
+
+      expect(response.status).to eq(422)
+      expect(response.parsed_body["errors"]).to contain_exactly(
+        "Reason is too long (maximum is 280 characters)",
+      )
+    end
+
     it "should create the right PM" do
       owner1 = Fabricate(:user, last_seen_at: Time.zone.now)
       owner2 = Fabricate(:user, last_seen_at: Time.zone.now - 1.day)