The expression of the cyclin-dependent kinase inhibitor p27, which negatively regulates cell cycle progression and functions as a tumour suppressor, is regulated at multiple levels including transcriptional, translational and posttranslational mechanisms. Moreover, the importance of noncoding RNAs in regulating p27 expression is rapidly emerging. Here we show that an intronic long noncoding RNA (lncRNA) that we name iLnc-p27 (intronic LncRNA regulating p27) plays a critical role in regulating p27 protein expression and cellular quiescence. We took advantage of an experimental system in which endogenous p27 and Ki67 proteins were visualised through dual genome-editing with green and red fluorescent proteins, respectively. Through RNA-seq we identified iLnc-p27 as the mostly increased lncRNA in p27highKi67low quiescent compared with p27lowKi67high proliferating cells. Our subsequent studies ascertained that iLnc-p27 is an intronic lncRNA derived from the second intron of CDKN1B, the gene encoding p27, and that the expression of iLnc-p27 is downregulated in diverse cancer types. Strikingly, although siRNA knockdown of iLnc-p27 did not impinge on the levels of p27 mRNA, it caused marked reduction in p27 protein expression that was not due to altered proteasomal degradation as shown in cycloheximide chase assays. On the other hand, overexpression of iLnc-p27 led to increase in p27 protein levels, although it similarly did not affect p27 mRNA expression. Moreover，the effects of iLnc-p27 on p27 mRNA and protein expression were confirmed in cells with iLnc-p27 knocked out using CRISPR/Cas9. These results point to the potential of iLnc-p27 in controlling translational regulation of p27. Functionally, iLnc-27 promoted cellular quiescence as evidenced by the decreased proportion of p27highKi67low quiescent cells induced by serum starvation or inhibition of survival signalling when iLnc-p27 was knocked down/out. In contrast, overexpression of iLnc-p27 promoted induction of p27highKi67low quiescent cells. Collectively, these results have identified the intronic lncRNA iLnc-p27 as an important regulator of cellular quiescence through facilitating p27 protein expression.