A tin-copper alloy electroplating bath which comprises a water-soluble tin salt, a water-soluble copper salt, an inorganic or organic acid or a water-soluble salt thereof, and one or more compounds selected from thioamide compounds and thiol compounds. The present invention makes it possible to form a tin-copper alloy deposit, in place of tin-lead alloy plating, on electronic parts such as chips, quartz crystal oscillators, hoops, connector pins, lead frames, bumps, lead pins of packages, and printed circuit boards.