Aims. We investigate the physical nature of the X-ray emitting source 1RXS J165443.5−191620 through optical photometry and time-resolved spectroscopy. Methods. Optical photometry is obtained from a variety of telescopes all over the world spanning  ≈27 days. Additionally, time-resolved spectroscopy is obtained from the MDM observatory. Results. The optical photometry clearly displays modulations consistent with those observed in magnetic cataclysmic variables: a low-frequency signal interpreted as the orbital period, a high-frequency signal interpreted as the white dwarf spin period, and an orbital sideband modulation. Our findings and interpretations are further confirmed through optical, time-resolved spectroscopy that displays Hα radial velocity shifts modulated on the binary orbital period. Conclusions. We confirm that 1RXS J165443.5−191620 is an intermediate polar with a spin period of 546 s and an orbital period of 3.7 h. In particular, 1RXS J165443.5−191620 is part of a growing subset of intermediate polars that display hard X-ray emission above 15 keV, white dwarf spin periods below 30 min, and spin-to-orbital ratios below 0.1.