diff --git a/docs/intro/overview.rst b/docs/intro/overview.rst
index 5937c5860..59c821175 100644
--- a/docs/intro/overview.rst
+++ b/docs/intro/overview.rst
@@ -29,6 +29,7 @@ https://quotes.toscrape.com, following the pagination:
 
     class QuotesSpider(scrapy.Spider):
         name = "quotes"
+        allowed_domains = ["quotes.toscrape.com"]
         start_urls = [
             "https://quotes.toscrape.com/tag/humor/",
         ]
diff --git a/docs/topics/spiders.rst b/docs/topics/spiders.rst
index 95c80d5dc..531f6324d 100644
--- a/docs/topics/spiders.rst
+++ b/docs/topics/spiders.rst
@@ -286,6 +286,7 @@ Spiders can access arguments in their `__init__` methods:
 
     class MySpider(scrapy.Spider):
         name = "myspider"
+        allowed_domains = ["www.example.com"]
 
         def __init__(self, category=None, *args, **kwargs):
             super().__init__(*args, **kwargs)
@@ -303,6 +304,7 @@ The above example can also be written as follows:
 
     class MySpider(scrapy.Spider):
         name = "myspider"
+        allowed_domains = ["www.example.com"]
 
         async def start(self):
             yield scrapy.Request(f"http://www.example.com/categories/{self.category}")
@@ -357,6 +359,7 @@ is automatically converted to an integer:
 
     class BookSpider(Args[MyParams], scrapy.Spider):
         name = "bookspider"
+        allowed_domains = ["books.toscrape.com"]
         start_urls = ["http://books.toscrape.com/catalogue"]
 
         async def start(self):
@@ -924,6 +927,7 @@ Simplest example: process all urls discovered through sitemaps using the
 
 
     class MySpider(SitemapSpider):
+        allowed_domains = ["www.example.com"]
         sitemap_urls = ["http://www.example.com/sitemap.xml"]
 
         def parse(self, response):
@@ -938,6 +942,7 @@ callback:
 
 
     class MySpider(SitemapSpider):
+        allowed_domains = ["www.example.com"]
         sitemap_urls = ["http://www.example.com/sitemap.xml"]
         sitemap_rules = [
             ("/product/", "parse_product"),
@@ -959,6 +964,7 @@ whose url contains ``/sitemap_shop``:
 
 
     class MySpider(SitemapSpider):
+        allowed_domains = ["www.example.com"]
         sitemap_urls = ["http://www.example.com/robots.txt"]
         sitemap_rules = [
             ("/shop/", "parse_shop"),
@@ -977,6 +983,7 @@ Combine SitemapSpider with other sources of urls:
 
 
     class MySpider(SitemapSpider):
+        allowed_domains = ["www.example.com"]
         sitemap_urls = ["http://www.example.com/robots.txt"]
         sitemap_rules = [
             ("/shop/", "parse_shop"),