diff --git a/README.md b/README.md
index 27440196e..00b8e76ae 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,8 @@ We've added comments to that file to highlight what each setting needs to be, bu
 - `proxy/ssl` and `proxy/host`: Kamal can set up SSL certificates for you automatically. To enable that, set the hostname again as `host`. If you don't want SSL for some reason, you can set `ssl: false` to turn it off.
 - `env/clear/MAILER_FROM_ADDRESS`: This is the email address that Fizzy will send emails from. It should usually be an address from the same domain where you're running Fizzy.
 - `env/clear/SMTP_ADDRESS`: The address of an SMTP server that you can send email through. You can use a 3rd-party service for this, like Sendgrid or Postmark, in which case their documentation will tell you what to use for this.
+- `env/clear/SINGLE_TENANT`: Set to `true` to enable single-tenant mode (disable multi-account signups).
+
 
 Fizzy also requires a few environment variables to be set up, some of which contain secrets.
 The simplest way to do this is to put them in a file called `.kamal/secrets`.
diff --git a/app/controllers/concerns/authorization.rb b/app/controllers/concerns/authorization.rb
index 81bd0c781..3b3e17a14 100644
--- a/app/controllers/concerns/authorization.rb
+++ b/app/controllers/concerns/authorization.rb
@@ -4,6 +4,7 @@ module Authorization
   included do
     before_action :ensure_can_access_account, if: -> { Current.account.present? && authenticated? }
     before_action :ensure_only_staff_can_access_non_production_remote_environments, if: :authenticated?
+    helper_method :single_tenant?
   end
 
   class_methods do
@@ -37,4 +38,8 @@ module Authorization
     def redirect_existing_user
       redirect_to root_path if Current.user
     end
+
+    def single_tenant?
+      ENV.fetch("SINGLE_TENANT", "false") == "true"
+    end
 end
diff --git a/app/controllers/signups_controller.rb b/app/controllers/signups_controller.rb
index 776421201..ec39063ce 100644
--- a/app/controllers/signups_controller.rb
+++ b/app/controllers/signups_controller.rb
@@ -3,6 +3,7 @@ class SignupsController < ApplicationController
   allow_unauthenticated_access
   rate_limit to: 10, within: 3.minutes, only: :create, with: -> { redirect_to new_signup_path, alert: "Try again later." }
   before_action :redirect_authenticated_user
+  before_action :prevent_signup_when_users_exist, if: -> { single_tenant? && User.any? }
 
   layout "public"
 
@@ -27,4 +28,8 @@ class SignupsController < ApplicationController
     def signup_params
       params.expect signup: :email_address
     end
+
+    def prevent_signup_when_users_exist
+      redirect_to new_session_url, alert: "Your account hasn’t been set up yet. Please request an invitation from one of your coworkers."
+    end
 end
diff --git a/app/models/signup.rb b/app/models/signup.rb
index 110c253ef..4409c589c 100644
--- a/app/models/signup.rb
+++ b/app/models/signup.rb
@@ -16,6 +16,7 @@ class Signup
   end
 
   def create_identity
+    return if single_tenant?
     @identity = Identity.find_or_create_by!(email_address: email_address)
     @identity.send_magic_link for: :sign_up
   end
@@ -96,4 +97,8 @@ class Signup
         attributes[:referrer]       = Current.referrer
       end
     end
+
+    def single_tenant?
+      ENV.fetch("SINGLE_TENANT", "false") == "true"
+    end
 end
diff --git a/app/views/sessions/magic_links/show.html.erb b/app/views/sessions/magic_links/show.html.erb
index 4e94f411e..810b3527a 100644
--- a/app/views/sessions/magic_links/show.html.erb
+++ b/app/views/sessions/magic_links/show.html.erb
@@ -3,7 +3,12 @@
 <div class="panel panel--centered flex flex-column gap-half <%= "shake" if flash[:alert] || flash[:shake] %>">
   <header>
     <h1 class="txt-x-large font-weight-black margin-none"><%= @page_title %></h1>
-    <p class="margin-none-block-start txt-medium">Then enter the verification code included in the email below:</p>
+    <p class="margin-none-block-start txt-medium">
+      <% if single_tenant? %>
+        iIf your email belongs to an existing account. 
+      <% end %>
+      Then enter the verification code included in the email below:
+    </p>
   </header>
 
   <%= form_with url: session_magic_link_path, method: :post, html: { data: { controller: "magic-link" } } do |form| %>
diff --git a/app/views/sessions/new.html.erb b/app/views/sessions/new.html.erb
index fc89bf681..643c9b66c 100644
--- a/app/views/sessions/new.html.erb
+++ b/app/views/sessions/new.html.erb
@@ -10,7 +10,11 @@
       </label>
     </div>
 
-    <p><strong>New here?</strong> <%= link_to "Sign up", new_signup_path %> to create an account. <strong>Already have an account?</strong> Enter your email and we’ll get you signed in.</p>
+    <% if single_tenant? %>
+      <p>Enter your email and we’ll get you signed in.</p>
+    <% else %>
+      <p><strong>New here?</strong> <%= link_to "Sign up", new_signup_path %> to create an account. <strong>Already have an account?</strong> Enter your email and we’ll get you signed in.</p>
+    <% end %>
 
     <button type="submit" id="log_in" class="btn btn--link center txt-medium">
       <span>Let’s go</span>
diff --git a/config/deploy.yml b/config/deploy.yml
index 8245a7136..1b2d274b7 100644
--- a/config/deploy.yml
+++ b/config/deploy.yml
@@ -30,6 +30,7 @@ env:
   clear:
     MAILER_FROM_ADDRESS: support@example.com # The email "from" address that Fizzy sends email from
     SMTP_ADDRESS: mail.example.com # The SMTP server you'll use to send email
+    SINGLE_TENANT: false # Set to true to enable single-tenant mode
     SOLID_QUEUE_IN_PUMA: true # Run background jobs in the app container
 
 
diff --git a/test/controllers/signups_controller_test.rb b/test/controllers/signups_controller_test.rb
index 17314ee90..c1b474c25 100644
--- a/test/controllers/signups_controller_test.rb
+++ b/test/controllers/signups_controller_test.rb
@@ -63,4 +63,19 @@ class SignupsControllerTest < ActionDispatch::IntegrationTest
       assert_redirected_to new_signup_completion_path
     end
   end
+
+  test "redirects to session#new when single_tenant and user exists" do
+    previous_single_tenant = ENV["SINGLE_TENANT"]
+    ENV["SINGLE_TENANT"] = "true"
+
+    users(:david)
+
+    untenanted do
+      get new_signup_path
+
+      assert_redirected_to new_session_url
+    end
+  ensure
+    ENV["SINGLE_TENANT"] = previous_single_tenant
+  end
 end