Isotopic analyses have proven to be an essential tool for obtaining more comprehensive and precise knowledge about past livestock strategies. Nevertheless, biogeochemical data for the Iron Age and, in particular, for the Iberian Peninsula are still very scarce. This study aims to provide a first and pivotal glimpse of sheep reproductive strategies adopted by north-eastern Iberian societies during the Middle and Late Iron Age, a period in which a process of urbanisation and agricultural expansion took place. Birth seasonality and the duration of the lambing period are here investigated through sequential oxygen isotope analyses performed on sheep’s second and third lower molars from four relevant Catalan sites (Mas Castellar de Pontós, Tossal de Baltarga, Sant Esteve d’Olius, Turó de la Rovira). These are contemporary (third century BC) and are located in different ecological and cultural areas. Results display diversified sheep reproductive patterns and distinct demographical management in the four settlements, with manipulations both on the season and on the duration of the lambing period. Thus, we propose that herd management and exploitation were determined by specific economic demands and the social organisation and environmental conditions of each area. Therefore, through this biogeochemical approach, we have been able to demonstrate that the resources, knowledge and time involved in the organisation of sheep husbandry were much more complex and planned than classical zooarchaeological studies have so far documented, highlighting the importance of livestock farming as a key activity in the economy of the Iron Age Iberian communities.