<p>Electrons bound to the surface of superfluid helium have been proposed as a scalable platform for charge- and spin-based quantum computing. Cavity quantum electrodynamics provides a promising method to implement quantum measurement and control of superfluid helium-bound electrons. In this approach, a cavity amplifies the strength of the interaction between the electron and a single photon for coherent exchange of quantum information and qubit readout. This strong coupling regime has been used for quantum measurement in different platforms such as superconducting qubits, atoms and semiconductor quantum dots. Here we demonstrate strong coupling between a microwave photon and the quantized motional state of a single electron on helium using a device comprising a quantum dot and superconducting resonator. Access to this regime provides a basis for developing single-electron spin qubit readout protocols using spin–orbit hybridization techniques that have already been demonstrated in semiconductor quantum dots.</p>