The 20-year-old has signed a one-year contract at the Riverside, and is likely to initially join up with the Under-23s.
Foster came through the ranks at Scunthorpe, where he made his first team debut when he was just 17, with Iron's boss at the time Keith Hill tipping the keeper to go on play for England.
He said: “It was a great moment for him. He’s an unbelievable character with a brilliant personality.
“He’s just so humble and such a good kid. He must have had a great upbringing.
"He’s calm and composed on the pitch. I’ve said it to the staff and I’ll say it to you now, he’ll play for England. There’s no question in my mind about that. He’s deserved his opportunity and kept plugging away, but he was always willing and able. It’s a privilege to work with people like him.”
Foster went on to play 13 times for Scunthorpe, with his impressive breakthrough catching the eye of Nottingham Forest, who took the young keeper on trial.
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He then returned to Scunthorpe before joining Hull City - his hometown club - two years ago.
Foster didn't make a first team appearance for the Tigers but spent time out on loan at Hartlepool and Torquay United last season, before linking up with [Boro](http://www.thenorthernecho.co.uk/sport/football/middlesbrough) on trial at the end of the campaign.
After catching the eye at Rockliffe, the young keeper has now earned a permanent deal.
It's set to be an interesting summer between the posts at Middlesbrough.
Sol Brynn is now fully fit after he was forced to miss the second half of last season with a dislocated shoulder, and the Teessider will be hoping to establish himself as No.1.
Seny Dieng also missed the second half of the season with a serious Achilles injury but posted a picture of his boots and gloves on Tuesday alongside the caption 'first time on pitch since surgery in January'.
Tom Glover has also reported back for pre-season. The Australian is moving into the final year of his contract, though the club have the option of an extra 12 months.
New boss Rob Edwards had his players out on the training pitches for the first time on Monday. Boro head out to Portugal this weekend for a week-long training camp, before their domestic pre-season campaign gets underway later this month.