We report the discovery of a large‐amplitude oscillation in the hot subdwarf B star CS 1246 and present multicolour photometry and time‐resolved spectroscopy supporting this discovery. We used the 0.41‐m Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes array to acquire data in the u′, g′, r′ and i′ filters simultaneously over three consecutive nights in 2009 April. These data reveal a single oscillation mode with a period of 371.707 ± 0.002 s and an amplitude dependent upon wavelength, with a value of 34.5 ± 1.6 mma in the u′ filter. We detected no additional frequencies in any of the light curves. Subsequently, we used the 4.1‐m SOAR telescope to acquire a time‐series of 248 low‐resolution spectra spanning 6 h to look for line profile variations. Models fits to the spectra give mean atmospheric values of Teff= 28 450 ± 700 K and log g= 5.46 ± 0.11 undergoing variations with semi‐amplitudes of 507 ± 55 K and 0.034 ± 0.009, respectively. We also detect a radial velocity oscillation with an amplitude of 8.8 ± 1.1 km s−1. The relationship between the angular and physical radii variations shows that the oscillation is consistent with a radial mode. Under the assumption of a radial pulsation, we compute the stellar distance, radius and mass as d=460 ±19090 pc, R= 0.19 ± 0.08 R⊙ and M= 0.39 ±0.300.13 M⊙, respectively, using the Baade–Wesselink method.