Kamara has joined Boro for an undisclosed fee from MLS side Sporting Kansas after spending a spell on loan at Norwich City last season.
He has agreed a two-year contract, with the option of a further year, and is relishing his move to the Championship club.
"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," Kamara told the club's official website.
"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 boss Tony Mowbray added: "We are delighted that Kei'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."
Mowbray has also brought in Butterfield from Norwich on a three-year contract with the option of a further 12 months.