Walcott wore Adidas' adiZero F50 football boots during the away game at Crystal Palace at the weekend. He moves from Adidas' arch rival Nike.
Walcott will also join existing Adidas football stars Luis Suaraz, Gareth Bale, James Rodriguez and Karim Benzema in the sports brand's current 'There will be haters' campaign.
Adidas has not said how long the deal will last, nor how much it cost.
Gregory Fernandez, Adidas director of sports marketing, said: "We are immensely proud to have signed Theo Walcott to become part of the growing Adidas family which includes some of the best players in the world.
"Theo is an exceptional player who has achieved so much in his career to date and we look forward to being part of that next chapter with him as one of the faces of the adidas F50."