Using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we report ion escape via the siphon effect within an open magnetic flux tube on Mars. MAVEN observed oppositely directed movements of hot solar wind protons and ionospheric ions, along with cold protons inside the flux tube, which can be explained by the siphoning escape mechanism. Collisions between hot solar wind protons and heavy ions in the lower Martian ionosphere transfer energy from the protons to the heavy ions, enabling their escape along with cooled protons through open magnetic field lines. This energy transfer reduces proton pressure within the flux tube, allowing continuous entry of hot solar wind protons that leads to ongoing ion loss. The high occurrence of open magnetic field lines in the Martian ionosphere suggests that siphoning ion escape could significantly contribute to the global ion loss of Mars.