Mathiassen is a surname and given name. Notable people with the surname include:
Erik Mathiassen Enge (1852–1933), Norwegian politician Samuel Mathiassen Føyn (1786–1854), Norwegian ship-owner and politician Idar Mathiassen (born 1976), Norwegian footballer Therkel Mathiassen (1892–1967), Danish archaeologist, anthropologist, cartographer and ethnographer Gotskalk Mathiassen Seim (1818–1873), Norwegian politician
See also Mathiassen Mountain, on Southampton Island in the Canadian territory of Nunavut