Neurons in the cerebral cortex are largely responsible for higher brain function in mammals. Although the maturation of neurons during development is important for neuronal functions, the molecular mechanisms underlying the maturation of neurons are not fully understood. In this study, we examined the role of the transcription factor Sox11 in the maturation of excitatory neurons in the cerebral cortex during development. We found that Sox11 expression was decreased in maturating neurons. Overexpression of Sox11 using in utero electroporation suppressed the expression of mature neuron markers and increased the expression of the immature neuron markers. Sox11 knockout mice showed enhanced expression of immature neuron markers. To understand the role of Sox11 in the morphological maturation of neurons, we used PASME (promoter-assisted sparse-neuron multiple-gene labeling using in utero electroporation) technique and found that Sox11 overexpression led to the retardation of morphological maturation of neurons. These results suggest that Sox11 inhibits the maturation of post-mitotic neurons in the cerebral cortex during development.