Modulating signaling pathways including Wnt and Hippo can induce cardiomyocyte proliferation in vivo. Applying these signaling modulators to human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in vitro can expand CMs modestly (<5-fold). Here, we demonstrate massive expansion of hiPSC-CMs in vitro (i.e., 100- to 250-fold) by glycogen synthase kinase-3β (GSK-3β) inhibition using CHIR99021 and concurrent removal of cell-cell contact. We show that GSK-3β inhibition suppresses CM maturation, while contact removal prevents CMs from cell cycle exit. Remarkably, contact removal enabled 10 to 25 times greater expansion beyond GSK-3β inhibition alone. Mechanistically, persistent CM proliferation required both LEF/TCF activity and AKT phosphorylation but was independent from yes-associated protein (YAP) signaling. Engineered heart tissues from expanded hiPSC-CMs showed comparable contractility to those from unexpanded hiPSC-CMs, demonstrating uncompromised cellular functionality after expansion. In summary, we uncovered a molecular interplay that enables massive hiPSC-CM expansion for large-scale drug screening and tissue engineering applications. Display omitted •GSK-3β inhibition-mediated hiPSC-cardiomyocyte proliferation is cell density dependent•GSK-3β inhibition with reduced cell-cell contact massively expands hiPSC-cardiomyocytes•LEF/TCF activity inhibits hiPSC-cardiomyocyte maturation without promoting cell cycling•Long-term expansion does not alter cardiomyocyte contractile function Deriving a large number of hiPSC-cardiomyocytes would be beneficial for large-scale tissue engineering and drug screening applications. Buikema et al. show that GSK-3β inhibition combined with removal of cell-cell contact enables massive expansion of hiPSC-cardiomyocytes with comparable function to non-expanded cells.