The 24-year-old is the second new signing announced by the Fir Park side, with Livingston defender Declan Gallagher also agreeing a move to Lanarkshire.
Motherwell boss Stephen Robinson told the club website: "We hope he will continue his creativity with us, having picked up the most assists in the Championship this season. Liam is a very talented boy and I still feel there are a lot of areas he will improve on with us."
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