  The youngest ($<$50 Myr) planets are vital to understand planet formation and
early evolution. The 17 Myr system HIP 67522 is already known to host a giant
($\simeq$10$R_\oplus$) planet on a tight orbit. In the discovery paper, Rizzuto
et al. 2020 reported a tentative single transit detection of an additional
planet in the system using TESS. Here, we report the discovery of HIP 67522 c
which matches with that single transit event. We confirm the signal with
ground-based multi-wavelength photometry from Sinistro and MuSCAT4. At a period
of 14.33 days, planet c is close to a 2:1 mean motion resonance with b (6.96
days or 2.06:1). The light curve shows distortions during many of the transits,
which are consistent with spot crossing events and/or flares. Fewer stellar
activity events are seen in the transits of planet b, suggesting that planet c
is crossing a more active latitude. Such distortions, combined with systematics
in the TESS light curve extraction, likely explain why planet c was previously
missed.
