The 24-year-old former England Under-21 international was a free transfer signing after passing a medical and agreeing personal terms.
Lito: “I'm raring to go. I haven't enjoyed the last two seasons one bit but this is a fresh start and I'm excited about the challenge.
“Other clubs were interested in signing me but there was only one place I wanted to go and that was Middlesbrough.
“The manager has been after me for about a year so it's great to feel wanted. I have a lot of respect for the gaffer and I want to do well for him and the club.
“I aim to repay him for his faith in me with goals. That's the main strength to my game and I'm looking forward to scoring goals for Middlesbrough.”