Using new palaeomagnetic and biostratigraphic data, we revise the age-model for the middle Eocene, deep-marine Aínsa Basin (Spanish Pyrenees), a tectonically active basin formed at a convergent-plate margin. This new age model provides a framework for evaluating the depositional history and sediment accumulation rates. New integrated magneto- and biostratigraphy data identify two normal and two reverse chrons of the geomagnetic polarity timescale (C21r, C21n, C20r, C20n) and place these Upper Hecho Group deposits in the middle Eocene (Lutetian). Nannofossil analysis identifies a biostratigraphic range from Subzone NP14b in the Gerbe System to Subzone NP15b at the top of the Aínsa System using key, age-diagnostic marker species such as Blackites inflatus, Blackites piriformis and Coccolithus gigas. We also present new nannofossil biostratigraphy from the Lower Hecho Group. This new Aínsa Basin chronostratigraphy enables inter-basinal correlations between the proximal fluvio-deltaic Tremp-Graus Basin and the more distal Jaca Basin, thereby providing a better understanding of the basin evolution.Supplementary materials: Field photographs of the sampled sections, magnetostratigraphic results, biostratigraphy results, alternative age model scenario and discussion on previous age model are available at https://doi.org/10.6084/m9.figshare.c.5083076