WR~21a was known as a massive spectroscopic binary composed of an O2.5 If*/WN6ha primary and an O3 V((f*))z secondary. Although a minimum value, the mass estimated for the primary placed it as one of the most massive stars found in our Galaxy. We report the discovery of photometric variations in the time series observations carried out by the Transiting Exoplanet Survey Satellite (TESS). These light variations are interpreted as formed by two main components: a sharp partial eclipse of the O3 secondary by the O2.5/WN6 star, and tidally excited oscillations. Based on the light minima a new ephemeris for the system is calculated. The system configuration is detached and the observed eclipse corresponds to the periastron passage. During the eclipse, the light curve shape suggests the presence of the heartbeat effect. The frequencies derived for the tidally excited oscillations are harmonics of the orbital period. Combining new and previously published radial velocity measurements, a new spectroscopic orbital solution is also obtained. Using the \textsc{phoebe} code we model the \textit{TESS} light curve and determine stellar radii of \(R_{\rm O2.5/WN6}=23.4\) R\(_\odot\) and \(R_{\rm O3}=14.3\) R\(_\odot\) and an orbital inclination \(i=62^\circ\!\!.2\pm0^\circ\!\!.9\). The latter combined with the spectroscopic minimum masses lead to absolute masses of \(M_{\rm O2.5/WN6}=93.2\) M\(_\odot\) and \(M_{\rm O3}=52.9\) M\(_\odot\), which establishes WR21a as belonging to the rare group of the very massive stars.