Downing, capped 35 times by England, steps down a division to re-join his home-town club in a deal worth ??5.5m that could rise to ??7m if Boro are promoted.
The 30-year-old graduated from the Middlesbrough academy and made 234 appearances for the club before leaving for Aston Villa at the end of the 2008-9 season.
Boro Head Coach Aitor Karanka was delighted to complete the signing of the player and he said: “It???s an amazing signing for us. He???s more than just a player.
“Nobody expected him to come here, but he wants to be here, and is completely committed to the club and the team. He is a very good person and I???m excited to work with him.”
Downing spent two years at Villa Park before joining Liverpool and then moved on to West Ham at the start of the 2013-14 campaign, where he played a regular role over the last two seasons.
Middlesbrough narrowly missed out on promotion under boss Aitor Karanka last season, as they were beaten by Norwich City in the Championship play-off final at Wembley in May.
Downing will now fly out to Marbella to link up with Karanka and the rest of the Middlesbrough squad.