Hearts boss Jim Jefferies previously worked with the 24-year old at Killie and was desperate to get him to Tynecastle Stadium on a free.
Jefferies confirmed the signing to Hearts News whilst the player himself was excited about a change of environment.
"Jamie is a player I know very well and I'm confident he'll be a good addition to the squad," Jefferies said.
"I am delighted to have brought him here and he will add competition for places which is good for any team."
Hamill added: "There was a lot of interest from other clubs but as soon as I heard of Hearts' interest, I wanted to speak to them as soon as possible.
"It's a massive club and Jim Jefferies gave me my chance at Kilmarnock.
"Billy Brown and Gary Locke are obviously people I know very well, too, so I am delighted to be joining a club of this stature.
"An added factor in my decision was European football and I'm looking forward to it.
"This is going to be the biggest challenge of my career."