Small-molecule inhibitors of dual-specificity tyrosine-regulated kinase 1A (DYRK1A) induce human beta cells to proliferate, generating a labeling index of 1.5%–3%. Here, we demonstrate that combined pharmacologic inhibition of DYRK1A and transforming growth factor beta superfamily (TGFβSF)/SMAD signaling generates remarkable further synergistic increases in human beta cell proliferation (average labeling index, 5%–8%, and as high as 15%–18%), and increases in both mouse and human beta cell numbers. This synergy reflects activation of cyclins and cdks by DYRK1A inhibition, accompanied by simultaneous reductions in key cell-cycle inhibitors ( CDKN1C and CDKN1A). The latter results from interference with the basal Trithorax- and SMAD-mediated transactivation of CDKN1C and CDKN1A. Notably, combined DYRK1A and TGFβ inhibition allows preservation of beta cell differentiated function. These beneficial effects extend from normal human beta cells and stem cell-derived human beta cells to those from people with type 2 diabetes, and occur both in vitro and in vivo. Keywords: diabetes, beta cell proliferation, regeneration, harmine, DYRK1A, TGFbeta, SMAD