SnSe alloy is examined for the first time as an anode for Na-ion batteries, and shows excellent electrochemical performance including a high reversible capacity of 707 mA h g(-1) and stable cycle performance over 50 cycles. Upon sodiation, SnSe is changed into amorphous NaxSn nanodomains dispersed in crystalline Na2Se, and SnSe is reversibly restored after desodiation.