Displacive phase transformations have been extensively studied, particularly related to advanced alloys such as high-strength transformation-induced-plasticity steels (TRIP steels) and shape memory alloys. Here, through compositional tuning of the stacking fault energy and hence phase stability, we introduce displacive phase transformation effects into a number of novel compositional complex alloys (CCAs) or high-entropy alloys (HEAs). We refer to these materials as TRIP-CCAs or TRIP-HEAs. The deformation-induced transformation from a stable face-centered cubic (FCC) matrix into a hexagonal close-packed (HCP) martensitic phase in the various CCAs upon tensile deformation was examined by multiple phase-sensitive microstructure probing methods. Compared to the single-phase CCAs, i.e. alloys without activation of deformation-induced phase transformation, the newly designed dual-phase CCAs exhibit significantly improved strength and ductility. Moreover, through the addition of interstitial elements and/or the modification of the alloys’ grain size, the phase transformation capacity was further tuned, thus further tailoring the corresponding mechanical properties.