A semiconductor device and an electro-optical device that ensures a stable output are provided even when there is a change in a source-drain current in a saturated operation region of a thin film transistor due to kink effects. The thin film transistor has a multi-gate structure with a polycrystalline silicon film as an active layer, and a source-side first thin film transistor portion and a drain-side second thin film transistor portion connected in series. The first thin film transistor portion has a drain-side back gate electrode that is connected with a first front gate electrode. The second thin film transistor portion has a source-side back gate electrode that is connected with a second front gate electrode.