Downing was one of West Ham's best players last season and even earned an England recall after shining at the tip of a central midfield diamond.
The 30-year-old has been moved on after the Hammers signed Marseille star playmaker Dimitri Payet for more than £10 million.
Premier League bores Downing
And the former Aston Villa and Liverpool man says it was time for a fresh challenge as he returns to his first club six years after leaving.
"If I stayed at West Ham, the aim would be to try and push into the top half of the table," Downing was reported as saying in the Mirror.
“Last season went pretty well for the club and we finished 12th, next season the aim will be to improve on that.
“Had I gone to another Premier League club from West Ham, the targets would be similar, but at Boro there’s a chance to achieve something really special."
Midfielder would have faced challenge to keep his place at West Ham
In truth Downing would have been under pressure to keep his place in the West Ham side under Slaven Bilic next season because the Hammers have been one of the busiest Premier League clubs in the summer transfer market spending nearly £25 million so far.
Co-owners David Sullivan and David Gold have been true to their word by investing significant funds to strengthen new boss Bilic's squad.
So far the East Londoners have brought in goalkeeper Darren Randolph on a free transfer, midfielders Pedro Obiang and Dimitri Payet for a combined total of around £15 million, defender Angelo Ogbonna for around £8 million and Carl Jenkinson in on loan again from Arsenal for a fee believed to be £2 million.
The Irons had already signed left back Stephen Hendrie from Scottish side Hamilton back in January and he officially became a West Ham player on July 1st.
Downing is the first player to be sold this summer by the Irons and represents a real coup for Boro who will be looking to challenge for promotion to the top flight again this season.