There is provided a welding jig for a secondary battery capable of preventing battery cells from floating in a jig by designing the jig so that the thinnest portion in a thickness direction of a battery cell is an alignment reference based on an end at which a terminal unit is positioned in as width direction of a battery cell. The welding jig for a secondary battery includes a main body including a battery cell inserting hole into which at least one battery cell is inserted and a jig inserting opening passing through the main body to expose a lower portion of the battery cell, a first side jig that is positioned on one side of the jig inserting opening of the main body and in which at least one first groove unit is formed to surround one side of an external surface of the battery cell, and a second side jig that is positioned on the other side of the jig inserting opening of the main body and in which at least one second groove unit is formed to surround the other side of the external surface of the battery cell. In a thickness direction of the battery cell, the first groove unit and the second groove unit are formed to be smaller than the battery cell inserting hole.