Self-propelling, light-activated colloidal particles can be actuated in water alone, but their performance is material-dependent.

It is demonstrated how the strength of activation for photocatalytic, self‐propelled colloids can be enhanced with a constant, uniform magnetic field. When exposed to ultraviolet light and hydrogen peroxide, the titanium dioxide‐based colloids become actively propelled. Due to the iron oxide core, a uniform field oriented perpendicular to the surface where motion takes place causes the asymmetrically shaped particles to rotate, which consequently leads to an increase in activity. The field‐dependent dynamics of self‐propulsion is quantified, and a qualitative description of how this effect arises is proposed. Since the application of the field is easily reversible, modulating the field on‐and‐off serves as a de facto “switch” that controls particle behavior.