Stabilization of a laser to an optical-clock transition has been demonstrated using Ramsey’s phase-modulation technique. The technique was implemented using a $674\mathrm{nm}$ laser and a component of the $5\mathit{s}{}^2\mathit{S}_{1∕2}\to 4\mathit{d}{}^2\mathit{D}_{5∕2}$ optical clock transition in a single ${}^{88}\mathrm{Sr}^{+}$ ion. The lock performance observed was consistent with the short-term stability of the local oscillator. Compared with existing frequency-modulation stabilization schemes, this technique offers advantages for use in single-ion optical clocks through its possible increased resolution and the ability to conveniently compensate known systematic shifts.