The Irishman will spend the final 12 months of his three-year Hearts contract at Fir Park.
He is keen to revitalise his career after two ill-fated seasons at Tynecastle Park, which saw him unable to establish himself and loaned out twice.
Sammon, 31, joined the Edinburgh club in June 2016 on a three-year deal after leaving Derby County.
However, he was loaned to Kilmarnock midway through his first season and then spent last year on loan at Partick Thistle.
The Glasgow club were relegated from the Ladbrokes Premiership in May and Sammon returned to Riccarton for pre-season training last month.
He was told he is not part of Levein’s first-team plans and has now agreed a move to Motherwell.
Sammon told Motherwell’s website: “I’m just looking forward to getting stuck in here, playing a lot of games, scoring a lot of goals, and just being a part of a successful season.
“I think the other strikers in the squad are similar. They’re physical, strong and quick, so I definitely think I can fit in here and I think my style of play will help me to have a good season here.
“And I’m looking forward to that, to training with the lads and to getting familiar with each other and how we play.”
Steelmen boss Stephen Robinson added: “I’m delighted to have added Conor to our squad. I’ve been keen to add another option in the striking department and he will fit our style of play perfectly.”