The top retailer is the first firm to announce its arrival at the site next to the Tesco Extra store and will create 75 jobs.
The news was unveiled by property firm Gallagher Developments at a consultation in Port Glasgow Library.
Bosses from B&Q spoke exclusively to the Telegraph to say they are delighted to be coming to Inverclyde.
Matthew Webster, their development chief, said: "We will be creating 75 full and part-time positions at all levels from experts, sales assistants and up to management level, and we are looking to recruit locally.
"We are very pleased to come to Port Glasgow - there is a strategic gap in our market with the nearest stores just now in Paisley and Great Western Road in Glasgow."
The site of the new retail park, above, covers 26,000 square metres and it boasts room for 13 non-food shops.
Work is ready to start in a year's time and the shopping centre is expected to be up and running by 2013.
The plans will be on display again today, until 5pm, at Port Glasgow Library.