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We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light‐curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the  J  band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type–colour relations, and over 9600 dwarfs ( J  &lt; 17) with spectral types later than K7 were found. The light curves of the late‐type sample are searched for periodicity using a Lomb–Scargle periodogram analysis. A total of 68 periodic variable M dwarfs are found in the sample with periods ranging from 0.16 to 90.33 d, with amplitudes in the range of ∼0.009 to ∼0.115 in the  J  band. We simulate active M dwarfs with a range of latitude‐independent spot coverages and estimate a periodically variable fraction of 1–3 per cent for stars where spots cover more than 10 per cent of the star's surface. Our simulated spot distributions indicate that operating in the  J  band, where spot contrast ratios are minimized, enables variability in only the most active of stars to be detected. These findings affirm the benefits of using the  J  band for planetary transit searches compared to visible bands. We also serendipitously find a Δ J  &gt; 0.2 mag flaring event from an M4V star in our sample.

J ‐band variability of M dwarfs in the WFCAM Transit Survey