Wieden & Kennedy beat Grey London in the final stage of a process that was handled by Oystercatchers.
Bartle Bogle Hegarty and Ogilvy & Mather were involved at an earlier stage.
Campaign revealed the review in June. The Brooklyn Brothers was the incumbent – it picked up the £20 million account in 2013 but did not repitch.
Mindshare will continue to handle media in Europe.
Deborah Dolce, the group brand and marketing director at TJX Europe, which owns TK Maxx, said: "Wieden & Kennedy’s strategic thinking and bold, creative work is outstanding and we look forward to this new partnership."
Earlier this week, TK Maxx released its Christmas campaign, "neighbour". The ad, created by Quiet Storm, encourages people to buy extra gifts for strangers in their community.
TK Maxx operates in the UK, Ireland, Germany and Poland. It launched a store in Austria earlier this year and plans to open in the Netherlands by the end of 2015.