We investigate the controlled K-type breakdown of a flat-plate boundary-layer with highly non-ideal supercritical fluid. Direct numerical simulations are performed at a Mach number of \(M_\infty =0.2\) for one subcritical (liquid-like regime) temperature profile and one strongly-stratified transcritical (pseudo-boiling) temperature profile with slightly heated wall. In the subcritical case, the formation of aligned \(\varLambda \) -vortices is delayed compared to the reference ideal-gas case of Sayadi et al. (J. Fluid Mech., vol. 724, 2013, pp. 480–509), with steady longitudinal modes dominating the late-transitional stage. When the wall temperature exceeds the pseudo-critical temperature, streak secondary instabilities lead to the simultaneous development of additional hairpin vortices and near-wall streaky structures near the legs of the primary aligned \(\varLambda \) -vortices. Nonetheless, transition to turbulence is not violent and is significantly delayed compared to the subcritical regime.