Androgen receptor (AR) that mediates androgen action is a crucial factor in male reproductive functions. Here, we report a novel AR corepressor ARR19 (androgen receptor corepressor-19 kDa), which has been isolated as a putative androgen-induced gene from murine testis. ARR19 encoding a leucine-rich protein is expressed only in male reproductive organs such as testis and prostate. ARR19 expression in the testis is developmentally regulated. Functional analysis conducted by the transient transfection of mammalian cells shows that ARR19 represses AR transactivation in a dose-dependent manner. Furthermore, yeast two-hybrid and glutathione S-transferase pull-down analyses reveal that ARR19 directly associates with AR through the N-terminal and leucine zipper-containing regions of ARR19 and the DNA binding-hinge domain of AR. Interestingly, ARR19 localized in the cytoplasmic compartment cotranslocates into the nucleus with AR upon androgen exposure. The ARR19 repression of AR transactivation is through the recruitment of histone deacetylase 4 (HDAC4) by ARR19. Overexpression of HDAC4 further inhibits the ARR19-repressed AR transactivation. In addition, ARR19 directly interacts with HDAC4 in vitro. Furthermore, DNA-protein complex immunoprecipitation assays reveal that HDAC4 is recruited to an androgen-regulated promoter through ARR19. Taken together, the results suggest that ARR19 may act as an AR corepressor in vivo and play an important role in male reproductive functions.