The striker joins the Jambos from Hibs and will link up with Andy Kirk's squad for pre-season training ahead of next month's season opener at Rangers.
Speaking to Hearts TV, Lia expressed her delight at joining the team ahead of their first top-flight campaign.
"I've been a Hearts fan my whole life so it's extra special to be stood here at Tynecastle.
"I spoke to Andy and I think he's a coach that i'm going to learn a lot from. We've got a big season coming up but i think we can be a tough team to beat and I'm looking forward to it."
Hearts Women Manager Andy Kirk added:
"We were keen to strengthen the squad in that area of the pitch and wanted someone with experience of playing at this level. Lia can score goals and I'm looking forward to working with her."
Source / Image: Heart of Midlothian FC