This paper presents a 5.8 GHz wearable rectangular ring antenna (RRA) with a metasurface that is constructed by a square split-ring resonator (SSRR) with an overlay square loop. The metasurface increases the directivity of the RRA to improve the sensing sensitivity of a bistatic self-injection-locked (SIL) Doppler radar for monitoring heart rate variability (HRV). The radar with the proposed antenna was first tested to detect a periodic movement of a copper plate and confirm a 77% improvement in detection amplitude due to the use of the metasurface. Then it was worn on the chest of a subject to measure his HRV. The extracted interbeat interval (IBI) from the radar measurements was compared to the electrocardiogram (ECG) results, showing a strong agreement with an average percentage root mean square error (PRMSE) of 6.26% or an average mean absolute percentage error (MAPE) of 3.90% throughout the experiments with different subjects.