The present work investigates the fire behaviour of a prestressed thin-walled concrete V-beam with variable cross section along its longitudinal axis. In particular, the results obtained from different modelling strategies implemented through analytical computations and Finite Element (FE) thermo-mechanical analyses conducted both with Euler–Bernoulli beam elements and with shell elements were compared. The models were able to properly allow for the prestressing force and its variation with temperature. The mode of failure was analysed both at ambient temperature and at elevated temperature by employing the ISO 834 standard heating curve as well as parametric fire curves so as to investigate the behaviour in the cooling phase. The outcomes showed that the beam model was not capable of accurately predicting the failure mode by largely overestimating the time of collapse. Indeed, the FE analysis with shell elements highlighted that beam behaviour assumptions with sections that remain plane were not satisfied as fire progressed. Furthermore, it also showed that the failure mode was not flexural in the longitudinal direction but due to the failure of the webs in transverse bending that interacted with longitudinal stresses. Finally, the analyses conducted by applying parametric fire curves highlighted possible failure in the cooling phase, which emphasises the need of careful analysis for these types of structural members.