Abstract Sound fisheries management requires knowledge of how the reproductive biology of targeted species varies over space and time and responds to external drivers. We determined the size at maturity and fecundity of the blue swimmer crab Portunus armatus (A. Milne-Edwards, 1861) (Brachyura: Portunidae) along the west coast of Australia at monthly, annual and interdecadal (1990s versus 2010s) time scales. The size at maturity of females determined by abdominal morphology (carapace width (CW) at 50% maturity (CW50)) was highest in sub-tropical Shark Bay at 26°S (104 mm), and declined latitudinally to temperate Geographe Bay at 34°S (93 mm). A notable exception was in a marine embayment at 32°S (Cockburn Sound), where CW50 was significantly lower than all other locations (83 mm). In contrast, batch fecundity was lower in sub-tropical (830,000 eggs at 127 mm CW) than temperate waters (950,000–990,000 eggs). Temporally, CW50 was significantly higher during January-June (typical growth period, encompassing the warmest months of the year) than during July-December (breeding period), and also differed between months, years, and decades. Positive correlations between water temperature, monthly and annual estimates of CW50, and functional maturity (size at first ovigery), were detected in most locations. Annual CW50 also increased with population density, particularly in locations where abundances and biomass were lowest. Results highlight plasticity in the reproductive biology of this portunid in response to environmental drivers such as temperature. Regular review of such parameters is important for ensuring robust fisheries management and stock assessments.