We present an exhaustive photometric and spectroscopic analysis of TOI-837, a F9/G0 35 Myr young star, hosting a transiting exoplanet, TOI-837 b, with an orbital period of ~8.32 d. Utilizing data from the Transiting Exoplanet Survey Satellite and ground-based observations, we determine a planetary radius of $0.818_{-0.024}^{+0.034}$ RJ for TOI-837 b. Through detailed High Accuracy Radial Velocity Planet Searcher spectroscopic time series analysis, we derive a Doppler semi-amplitude of $34.7_{-5.6}^{+5.3}$ ${\rm m\, s^{-1}}$, corresponding to a planetary mass of $0.379_{-0.061}^{+0.058}$ MJ. The derived planetary properties suggest a substantial core of approximately 70 M⊕, constituting about 60 per cent of the planet's total mass. This finding poses a significant challenge to existing theoretical models of core formation. We propose that future atmospheric observations with JWST could provide insights into resolving ambiguities of TOI-837 b, offering new perspectives on its composition, formation, and evolution.