Retail chain agrees to be an advertising partner for the launch of the website.
The advertising deal with B&Q includes display advertisements and advertorial features, both online and in the magazine.
It will also highlight B&Q gardens from BBC Gardeners' World Live at The NEC, a five-day garden show held in June.
The site - gardenersworld.com - has been designed to become an online destination for UK gardeners.
It offers daily blogs, a forum, weather forecasts and gardening advice.