Peltier has signed a one-year deal and becomes the fourth signing of the summer, to date.
Peltier, a product of Liverpool’s Academy, has made over 500 career appearances and was a promotion winner with Neil Warnock’s Cardiff City side in 2018.
Boro boss Warnock said: “I know what Lee can do.
Paul Hartley reveals Hartlepool United transfer latest following arrival of ex-Birmingham City and Walsall winger
Ex-Birmingham City and Walsall winger in line for Hartlepool United debut against Tranmere Rovers
Chris Wilder makes Middlesbrough striker admission
Ex-Nottingham Forest defender starts for Tranmere Rovers on his return to Hartlepool United
Hartlepool United to miss out on Leeds United midfielder with 19-year-old set for Motherwell switch
"He’s fantastic in the dressing room, and we haven’t got many senior pros in there.
“He can play in a lot of positions – both full backs, centre half, centre midfield… he’s played everywhere for me over the years. I’m aware inside and out what he can do.
“Darnell Fisher has picked up an injury which we’re not sure how serious it is at this stage so we are thin down that side of the park, and in Pelts I just know what a good defender he is and what he will bring to us.”
Peltier’s career has taken in Huddersfield Town, Leicester City, Leeds United and Cardiff where he made 163 appearances and played a pivotal role in their promotion to the Premier League.
Earlier on Friday it was announced Boro had signed Uche Ikpeazu.
The 26-year-old completed the formalities of his move ahead of the start of pre-season training after Boro agreed a deal with Wycombe for his services. He has signed a three-year contract.
The fee is reported to be worth £1m in total but that includes various add-ons.
Boro boss Neil Warnock said: “I’ve always liked Uche back to when he was at Hearts and I watched him a few times.
"He looks raw at times but he’s so keen and dedicated wherever he plays and I think the fans will really get behind him because he’s such a 100% player.
“I was really impressed in my meetings with him with his dedication and desire to do well for Middlesbrough.
"I’m really looking forward to working with him.”
The striker was a product of Reading’s Academy before he went on to join Watford.
Loan spells with Crewe, Doncaster, Port Vale, and Blackpool all followed before a two-season spell at Cambridge produced 22 goals in 80 appearances.