Extracellular microparticles (MPs) can function as drug-delivery vehicles for anticancer drugs. Here, we show that the softness of MPs derived from tumour-repopulating cells (TRCs) isolated from three-dimensional fibrin gels enhances the MPs' drug-delivery efficiency. We found that, compared with MPs derived from tumour cells cultured in conventional tissue-culture plastic, TRC-derived MPs intravenously injected in tumour-xenograft-bearing mice showed enhanced accumulation in tumour tissues, enhanced blood-vessel crossing and penetration into tumour parenchyma, and preferential uptake by highly tumorigenic TRCs. We also show that the cytoskeleton-related protein cytospin-A plays a critical role in the regulation of TRC-derived MP softness. The modulation of the mechanical properties of TRC-derived MPs could aid the efficiency of delivery of anticancer drugs.