The 21-year-old defender arrives with two-and-a-half years of Championship experience gained during loan spells at Stoke City and Plymouth Argyle.
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Phillips, who is also an England Under-21 international, attracted significant interest this summer but chose Middlesbrough as his next destination, ending his three-year association with Spurs.
“I’m very excited to be here and can’t wait to be on the pitch,” Phillips said.
“This is an amazing club and it deserves to be in the Premier League.
“I hope to be a good addition to what’s already a fantastic squad, and hopefully we can achieve great things together this season.“