The corrosion behavior of as-cast Mg–5Sn–xIn (x = 0, 1, 2, 3, 4, all in wt%) alloys in 3.5 wt% NaCl solution was investigated using weight loss, hydrogen evolution and electrochemical test. The results showed that the microstructures of all alloys consisted of typical coarse dendrites without the presence of any In-rich phase. The corrosion resistance of Mg–5Sn–xIn alloys was enhanced monotonously with increasing In content up to 3 wt%. The electrochemical measurements revealed that such improvements could be mainly attributed to the retardation of cathodic reaction, decreasing the rate of hydrogen evolution.