Marie Lacoste Gérin-Lajoie (October 19, 1867 in Montreal – November 1, 1945 in Montreal) was a pioneer Quebec feminist who co-founded the Fédération nationale Saint-Jean-Baptiste with Caroline Dessaulles-Béique (in 1907), an organization which campaigned for social and political rights for women.
In addition to its legal work, the Federation nationale also championed social causes such as providing milk for children and mothers, fighting alcoholism and illness, raising awareness of infant mortality, and various other issues that affected women's lives.