The 29-year-old joins Boro from MLS side Sporting Kansas City and pens a two-year contract with the option of a further year.
Boss Tony Mowbray told the club's official website: "We are delighted that Kei's s here. He will add to our attacking options all across the front line.
"He'll bring some added power, pace and an aerial threat to our squad."
Kamara added: "I played a game on Saturday, got on a plane on Sunday and now I'm here on Monday so it's been a long couple of days but now I'm delighted to be here.
"I've heard playing in the Championship is a lot harder than the Premier league but I'm always ready for a challenge so I'm looking forward to it. Hopefully my own style and way of playing will help the team out."
Boro have further added to their squad with the late capture of Norwich midfielder Jacob Butterfield for an undisclosed fee.
The 23-year-old, who joined the Canaries in 2012 from Barnsley, has penned a three-year deal with the option of a further year.