The supermarket, which acquired Argos and Habitat last year, is also set to launch a new homeware brand, Sainsbury’s Home.
Tu is the UK’s sixth largest clothing retailer by volume. Portas has been tasked with establishing it as "a destination brand for high-street fashion".
Sainsbury's called the review in November last year in a process run by AAR. Portas pitched against Seven, The Corner, and Y&R London.
Mark Given, marketing director of Sainsbury’s, said: "Having spent a large proportion of 2016 sharpening up our advertising strategy in this very competitive sector, 2017 is set to be a very exciting year for us.
"The work produced by Portas during the pitch process was very much in line with this strategy and it was great to see a refreshing new take on our fashion and home communications."
General merchandise advertising was previously handled by Abbott Mead Vickers BBDO as part of the main Sainsbury’s account, but the retailer separated it from grocery last year.
Sainsbury's appointed Wieden & Kennedy to its grocery account last August and the first work from the independent shop emerged last month.
Earlier this month, M/SIX picked up the £100m Sainsbury’s media account. PHD had held the business for 22 years.
Gravity Road created an online ad to promote Tu’s Christmas jumpers on a project basis in 2014 and today is the supermarket's retained digital content agency for the food business.