We present Hα-derived star formation rates (SFRs) for the galaxy cluster Cl J0023+0423B at z = 0.845. Our 3 σ flux limits correspond to an SFR of 0.24 h M☉ yr-1, and our minimum reliable Hα + [N ] equivalent width is greater than 10 Å, demonstrating that near-infrared narrowband imaging can sample the star-forming galaxy population in distant clusters. Comparison with spectroscopy shows that the number of false detections is low (9% ± 6%) and that our Hα equivalent widths are correlated with spectroscopically determined [O II] equivalent widths. A magnitude-limited spectroscopic survey conducted over the same area missed 70% of the star-forming galaxies and 65% of the integrated star formation. Using Hubble Space Telescope Wide Field Planetary Camera 2 Archive images, we fit Sersic profiles to all galaxies with significant narrowband equivalent widths and find that equivalent width decreases as the steepness of galaxy profile increases. We find no significant population of early-type galaxies with ongoing star formation. The integrated SFR per cluster mass of Cl J0023+0423B is a factor of 10 higher than that of the three z ~ 0.2 clusters in the literature with available Hα observations. A larger sample of z ~ 0.8 clusters spanning a range of cluster masses is needed to determine whether this variation is due to a difference in cluster mass or redshift.