Mammalian genomes encode numerous antisense non-coding RNAs, which are assumed to be involved in the regulation of the sense gene expression. However, the mechanisms of their action and involvement in the development of diseases have not been well elucidated. The ANA/BTG3 protein is an antiproliferative protein whose expression is downregulated in prostate and lung cancers. Here we show that an antisense transcript of the ANA/BTG3 gene, termed ASBEL, negatively regulates the levels of ANA/BTG3 protein, but not of ANA/BTG3 mRNA and is required for proliferation and tumorigenicity of ovarian clear cell carcinoma. We further show that knockdown of ANA/BTG3 rescues growth inhibition caused by ASBEL knockdown. Moreover, we demonstrate that ASBEL forms duplexes with ANA/BTG3 mRNA in the nucleus and suppresses its cytoplasmic transportation. Our findings illustrate a novel function for an antisense transcript that critically promotes tumorigenesis by suppressing translation of the sense gene by inhibiting its cytoplasmic transportation.