The three-dimensional flow transition is studied in the wake of a heated diamond-shaped cylinder using air (Prandtl number, Pr = 0.7) as a working fluid. The cylinder is exposed to free-stream crossflow at an angle perpendicular to gravity. In this study, a non-Oberbeck–Boussinesq compressible model is used to capture the changes in transport properties and thermal straining of the fluid particles on large-scale heating. The heating level is defined as over-heat ratio ε = (T_w − T_∞)/T_∞, where T_w and T_∞ are the uniform surface and the surrounding temperatures. A flux-based particle velocity upwind-modified+ (PVU-M+) scheme is employed to solve compressible flow governing equations (in a body-fitted coordinate system). All computations are performed at a low Mach number (M = 0.1). For model validation, the obtained values of the time-averaged drag coefficient (C_D) and the Strouhal number (St) are compared with the values reported in the literature at Reynolds numbers Re = 100 − 200 and ε = 0. In this investigation, it is observed that the buoyancy effect increases by increasing the ε value, and the flow field around the cylinder becomes asymmetric. At Re = 200, significant changes in the vorticity structure shape and their spanwise wavelength are observed in the cylinder wake for ϵ values ranging from 0 to 1. The heating effects are observed in the time histories of global flow parameters such as force coefficients (C_D and C_L) and Nusselt number (Nu).