Reynolds number efects on the aerodynamics of the moderately thick NACA 0021 airfoil were experimentally studied by means of surface-pressure measurements. The use of a high-pressure wind tunnel allowed for variation of the chord Reynolds number over a range of 5.0 × 105 ≤ Rec ≤ 7.9 × 106. The angle of attack was incrementally increased and decreased over a range of 0◦ ≤ α ≤ 40◦, spanning both the attached and stalled regime at all Reynolds numbers. As such, attached and separated conditions, as well as the static stall and reattachment processes were studied. A fundamental change in the fow behavior was observed around Rec = 2.0 × 106. As the Reynolds number increased beyond this value, the stall type gradually shifted from trailing-edge stall to leading-edge stall. The stall angle and the maximum lift coefcient increased with Reynolds number. Once the fow was separated, the separation point moved upstream, and the suction peak decreased in magnitude with increasing Reynolds number. Two distinct types of hysteresis in reattachment were observed. The data from this study are publicly available at https://doi.org/10.34770/9mv0-zd78.