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T‐cell development is a spatially and temporally regulated process, orchestrated by well‐defined contributions of transcription factors and cytokines. Here, we identify the noncoding RNA miR‐142 as an additional regulatory layer within murine thymocyte development and proliferation. MiR‐142 deficiency impairs the expression of cell cycle‐promoting genes in mature mouse thymocytes and early progenitors, accompanied with increased levels of cyclin‐dependent kinase inhibitor 1B (Cdkn1b, also known as p27^Kip1). By using CRISPR/Cas9 technology to delete the miR‐142‐3p recognition element in the 3’UTR of cdkn1b, we confirm that this gene is a novel target of miR‐142‐3p in vivo. Increased Cdkn1b protein expression alone however was insufficient to cause proliferation defects in thymocytes, indicating the existence of additional critical miR‐142 targets. Collectively, we establish a key role for miR‐142 in …