The 21-year-old has left West Ham United to sign a four-year deal and become big-spending Boro’s sixth signing of the summer.
Hammers manager Slaven Bilic admitted he wanted Fletcher to stay at the London Stadium but that the offer had been too “fantastic” to turn down.
Fletcher moved to West Ham a year ago from Manchester United after making his name when helping Barnsley to the double of promotion from League One and the Johnstone’s Paint Trophy in 2016.
Before Boro confirmed the signing, Ipswich Town had enquired about taking the striker on loan.
“There was me, like a pillock, ringing up trying to take him on loan,” said Tractor Boys manager Mick McCarthy.
“I have tried for him in a couple of windows and was only speaking to his agent yesterday. He must have been laughing right up his sleeve.”
Fletcher joins Britt Assombalonga, Martin Braithwaite, Cyrus Christie, Jonny Howson and Darren Randolph in moving to the Riverside since Garry Monk’s appointment last month.
Another striker on the move yesterday in the Championship was Lee Erwin, who has completed a permanent move from Leeds United to Kilmarnock.
The 23-year-old made little impression at Elland Road after joining from Motherwell during the summer of 2015. He failed to score and made just two starts.
Erwin did, though, finish last season as top scorer at Oldham Athletic after moving to Boundary Park on loan in July, 2016. He has signed a two-year contract with Kilmarnock.
York Council chiefs, meanwhile, have approved plans for the new 8,000 capacity community stadium and work is set to begin in October with York City due to move in ahead of the 2019-20 season.