-Methyladenosine (m A) is the most abundant modification of mammalian mRNAs. RNA methylation fine tunes RNA stability and translation, altering cell fate. The fat mass- and obesity-associated protein (FTO) is an m A demethylase with oncogenic properties in leukemia. Here, we show that expression is suppressed in ovarian tumors and cancer stem cells (CSC). FTO inhibited the self-renewal of ovarian CSC and suppressed tumorigenesis , both of which required FTO demethylase activity. Integrative RNA sequencing and m A mapping analysis revealed significant transcriptomic changes associated with overexpression and m A loss involving stem cell signaling, RNA transcription, and mRNA splicing pathways. By reducing m A levels at the 3'UTR and the mRNA stability of two phosphodiesterase genes ( and ), FTO augmented second messenger 3', 5'-cyclic adenosine monophosphate (cAMP) signaling and suppressed stemness features of ovarian cancer cells. Our results reveal a previously unappreciated tumor suppressor function of FTO in ovarian CSC mediated through inhibition of cAMP signaling. SIGNIFICANCE: A new tumor suppressor function of the RNA demethylase FTO implicates m A RNA modifications in the regulation of cyclic AMP signaling involved in stemness and tumor initiation.