Demethylation by the AlkB dioxygenases represents an important mechanism for repair of N-alkylated nucleotides. However, little is known about their functions in mammalian cells. We report the purification of the ALKBH3 complex and demonstrate its association with the activating signal cointegrator complex (ASCC). ALKBH3 is overexpressed in various cancers, and both ALKBH3 and ASCC are important for alkylation damage resistance in these tumor cell lines. ASCC3, the largest subunit of ASCC, encodes a 3′-5′ DNA helicase, whose activity is crucial for the generation of single-stranded DNA upon which ALKBH3 preferentially functions for dealkylation. In cell lines that are dependent on ALKBH3 and ASCC3 for alkylation damage resistance, loss of ALKBH3 or ASCC3 leads to increased 3-methylcytosine and reduced cell proliferation, which correlates with pH2A.X and 53BP1 foci formation. Our data provide a molecular mechanism by which ALKBH3 collaborates with ASCC to maintain genomic integrity in a cell-type specific manner. Display omitted ► ALKBH3, a DNA/RNA dealkylating enzyme, associates with ASCC, an uncharacterized complex ► The largest subunit of ASCC, ASCC3, encodes a 3′-5′ DNA helicase ► ALKBH3 and ASCC3 promote dealkylation repair on double-stranded DNA ► Both ALKBH3 and ASCC3 are important for proliferation and dealkylation repair in specific tumor cell lines