A fundamental question in neurobiology is how the balance between proliferation and differentiation of neuronal precursors is maintained to ensure that the proper number of brain neurons is generated. Substantial evidence implicates DYRK1A (dual specificity tyrosine-phosphorylation-regulated kinase 1A) as a candidate gene responsible for altered neuronal development and brain abnormalities in Down syndrome. Recent findings support the hypothesis that DYRK1A is involved in cell cycle control. Nonetheless, how DYRK1A contributes to neuronal cell cycle regulation and thereby affects neurogenesis remains poorly understood. In the present study we have investigated the mechanisms by which DYRK1A affects cell cycle regulation and neuronal differentiation in a human cell model, mouse neurons, and mouse brain. Dependent on its kinase activity and correlated with the dosage of overexpression, DYRK1A blocked proliferation of SH-SY5Y neuroblastoma cells within 24 h and arrested the cells in G₁ phase. Sustained overexpression of DYRK1A induced G₀ cell cycle exit and neuronal differentiation. Furthermore, we provide evidence that DYRK1A modulated protein stability of cell cycle-regulatory proteins. DYRK1A reduced cellular Cyclin D1 levels by phosphorylation on Thr286, which is known to induce proteasomal degradation. In addition, DYRK1A phosphorylated p27^Kip1 on Ser10, resulting in protein stabilization. Inhibition of DYRK1A kinase activity reduced p27^Kip1 Ser10 phosphorylation in cultured hippocampal neurons and in embryonic mouse brain. In aggregate, these results suggest a novel mechanism by which overexpression of DYRK1A may promote premature neuronal differentiation and contribute to altered brain development in Down syndrome.