The 25-year old English-born Jamaican international has been with the Hornets for his entire career up until now but has decided to make the move to the Maedjski Stadium ahead of the Royals' first season back in the top flight since 2008.
"It's a great feeling. The Premier League is the best league in the world and I'm delighted to have the oportunity to try to work my way into this team and perform for Reading Football Club," the man they call 'Mapps' told Reading FC Player.
"They were three guys who I worked with coming up through the youth system at Watford, and I've played with Jobi and Bryn Gunnarsson as well. So there are a few ties there from back in my old Watford days.
"So it was nice to know there would be a few familiar faces here. But Reading Football Club is a great place to come to anyway, so it wasn't a hard decision to make the move here.
"I just can't wait to get onto a training pitch with all the lads and start settling in, getting to know everyone and playing in the pre-season games we have coming up."