L-Sox5 is a member of sex-determining region Y-type high mobility group box (SOX) family of transcription factors. We assessed the effects of retroviral overexpression of L-Sox5 on chondrocyte differentiation using the clonal murine cell line ATDC5. We observed a temporal-restricted expression pattern of L-Sox5 in insulin-induced ATDC5 cells differentiating toward chondrocyte lineage. The protein expression levels of L-Sox5 showed a drastic decrease in contrast to unaltered mRNA levels during differentiation. L-Sox5 delayed the differentiation of ATDC5 cells as evidenced by Alcian blue staining for proteoglycan synthesis. The mRNA levels of chondrocyte and hypertrophic/osteoarthritic markers were markedly decreased or delayed in L-Sox5 overexpressing cells. L-Sox5 abrogated the promoter activity of Runx2. These results suggest that L-Sox5 protein expression may diminish along with the progress of chondrogenic differentiation. L-Sox5 may act as a negative regulator if expressed aberrantly at least in part by regulating the critical fate of chondrogenesis.