We propose and demonstrate an all-optical approach to single-electron sensing using the optical transitions of a semiconductor quantum dot. The measured electric-field sensitivity of 5 (V/m)/root Hz corresponds to detecting a single electron located 5 mu m from the quantum dot-nearly 10 times greater than the diffraction limited spot size of the excitation laser-in 1 s. The quantum-dot-based electrometer is more sensitive than other devices operating at a temperature of 4.2 K or higher and further offers suppressed backaction on the measured system.