N⁶-methyladenosine (m⁶A) is the most prevalent internal modification of messenger RNA (mRNA) in higher eukaryotes. Here we report ALKBH5 as another mammalian demethylase that oxidatively reverses m⁶A in mRNA in vitro and in vivo. This demethylation activity of ALKBH5 significantly affects mRNA export and RNA metabolism as well as the assembly of mRNA processing factors in nuclear speckles. Alkbh5-deficient male mice have increased m⁶A in mRNA and are characterized by impaired fertility resulting from apoptosis that affects meiotic metaphase-stage spermatocytes. In accordance with this defect, we have identified in mouse testes 1,551 differentially expressed genes that cover broad functional categories and include spermatogenesis-related mRNAs involved in the p53 functional interaction network. The discovery of this RNA demethylase strongly suggests that the reversible m⁶A modification has fundamental and broad functions in mammalian cells.