V375 Cassiopeia (V375 Cas) is an early B-type close binary with an orbital period of 1.4734273 d. Based on high-quality and continuous light curves from the Transiting Exoplanet Survey Satellite (TESS) and new low-resolution spectra, we have determined photometric solutions of the eclipsing binary using the Wilson-Devinney method. We have found that V375 Cas is a semidetached binary where the secondary component fills the critical Roche lobe. Meanwhile, its O - C diagram was constructed with the data spanning 122 yr. An upward parabolic variation is discovered to be superimposed on a cyclic modulation with a semi-amplitude of 0.0938 (+/- 0.0143) d and a period of 118.74 (+/- 12.87) yr. The parabolic change reveals a period increase at a rate of d(p)/dt = +5.42 x 10(-7) d yr(-1). Both the semidetached configuration and the period increase suggest that V375 Cas is undergoing a late case A mass transfer from the less-massive component to the more-massive component. The cyclic change is attributed to be the effect of the light travel time via the presence of a third body. The minimum mass of the tertiary companion is estimated as M-3 = 4.24 (+/- 0.87) M-circle dot, which orbits around the central eclipsing binary with a nearly circular orbit (e = 0.184). It is detected that the light contribution of the third body is about 8.0 per cent, indicating that it is a massive main-sequence star. All the results reveal that V375 Cas is a hierarchical triple system where a massive main-sequence star accompanies a massive semidetached mass-transfer binary at an orbital separation of 60.3 au.