The experienced ex-Leeds captain, 34, who worked with Boro manager Neil Warnock at Elland Road and Cardiff City, has signed a one-year deal after arriving from West Brom.
His arrival follows on from the acquisition of Wycombe Wanderers striker Uche Ikpeazu.
On Peltier, promoted at Cardiff during the Warnock era, the Boro chief said: “I know what Lee can do. 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.”