Covalent modifications of histones play an important role in chromatin architecture and dynamics. In particular, histone lysine methylation is important for transcriptional control during diverse biological processes. The nuclear protein Jmjd5 (also called Kdm8) is a histone lysine demethylase that contains a JmjC domain in the C-terminal region. In this study, we have generated Jmjd5-deficient mice (Jmjd5^Δ/Δ) to investigate the in vivo function of Jmjd5. Jmjd5^Δ/Δ embryos showed severe growth retardation, resulting in embryonic lethality at the mid-gestation stage. Mouse embryonic fibroblasts (MEFs) derived from Jmjd5 hypomorphic embryos (Jmjd5^neo/neo) also showed the growth defect. Quantitative PCR analysis of various cell cycle regulators indicated that only Cdkn1a expression was upregulated in Jmjd5^neo/neo MEFs and Jmjd5^Δ/Δ embryos. A knockdown assay with Cdkn1a-specific small interfering RNAs revealed that the growth defect of Jmjd5^neo/neo MEFs was significantly rescued. In addition, a genetic study using Jmjd5^Δ/Δ; Cdkn1a^Δ/Δ double-knockout mice showed that the growth retardation of Jmjd5^Δ/Δ embryos was partially rescued by Cdkn1a deficiency. Chromatin immunoprecipitation analysis showed that increased di-methylated lysine 36 of histone H3 (H3K36me2) and reduced recruitment of endogenous Jmjd5 were detected in the transcribed regions of Cdkn1a in Jmjd5^neo/neo MEFs. Taken together, these results suggest that Jmjd5 physiologically moderates embryonic cell proliferation through the epigenetic control of Cdkn1a expression.