Fast radio bursts (FRBs) are bright, millisecond-duration radio transients originating from sources at extragalactic distances(1), the origin of which is unknown. Some FRB sources emit repeat bursts, ruling out cataclysmic origins for those events(2-4). Despite searches for periodicity in repeat burst arrival times on timescales from milliseconds to many days(2,5-7), these bursts have hitherto been observed to appear sporadically and-although clustered(8)-without a regular pattern. Here we report observations of a 16.35 +/- 0.15 day periodicity (or possibly a higher-frequency alias of that periodicity) from the repeating FRB 180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project(4,9). In 38 bursts recorded from 16 September 2018 to 4 February 2020 utc, we find that all bursts arrive in a five-day phase window, and 50 per cent of the bursts arrive in a 0.6-day phase window. Our results suggest a mechanism for periodic modulation either of the burst emission itself or through external amplification or absorption, and disfavour models invoking purely sporadic processes.