The 32-year-old becomes Boro's ninth signing of the summer and provides cover on either flank after a frustrating final campaign with the Whites, where he made just five appearances following Leeds' promotion to the Premier League.
Byram made 80 appearances for Leeds across his second spell at Elland Road and was part of the squad that won the Championship title in May 2025, though his game time diminished sharply once the club returned to the top flight.
Norwich City had also shown interest in re-signing their former full-back, but it is Middlesbrough who have won the race, helped in part by head of football Kieran Scott and head of recruitment Chris Jones both knowing Byram well from their time together at Carrow Road.
"I'm buzzing to be here," Byram said. "It's a club I know well from playing against them over many seasons, so I'm very familiar with the place and delighted to be here. It's a great club and a big club, and one that I'm really happy to be part of."
A product of Leeds' academy, Byram made almost 150 appearances for the Whites before his first move to the Premier League with West Ham United in January 2016. He went on to represent Norwich and Nottingham Forest, as well as a second spell with Leeds, and has now made more than 300 career appearances across the Premier League and Championship.
"I've been around this league for a number of years and had a couple of promotions in that time, so I know what it takes," he added. "It's an incredibly tough league and a long season, but it's a great squad and great group. If I can add a bit of experience and a bit of quality, that's what I'm here to do."
Byram is naturally right-footed and will primarily compete with Callum Brittain for the right-back spot under head coach Kim Hellberg, though his versatility means he can also cover on the left when required.
The move continues a busy summer at the Riverside Stadium, with Middlesbrough among the Championship's biggest spenders as they push for a return to the Premier League. Head of football Kieran Scott has spoken openly about the club's determination to remain aggressive in the closing weeks of the window, with further reinforcements still being considered in midfield and up front.