Human administrative borders have no effect on wild animals, and the vast home ranges of large carnivores often cause them to live simultaneously on the territory of two or more countries or jurisdictions with different management policies. Here, we investigate the importance of transboundary population monitoring using as a case study the Pyrenean brown bear population (Ursus arctos) that lives in France, Andorra and Spain. Using capture-recapture models and the Pollock's robust design, we estimated abundance and demographic parameters using data collected separately in France and Spain and a dataset gathered from joint monitoring on both sides of the border. As expected, the abundance estimates from French (from 11 bears in 2008 to 13 in 2014) or Spanish (from 4 bears in 2008 to 9 in 2014) data only were lower than abundance obtained from both sides of the border (from 11 in 2008 to 18 in 2014). The joint monitoring dataset also highlighted the importance of individual detection heterogeneity that, if ignored, would lead to underestimation. Our results reinforce the importance of transboundary cooperation when dealing with animal populations with territory spanning two or more administrative jurisdictions for collecting reliable scientific data and providing relevant abundance estimation to take sound management decisions.