This study presents an extensive geochemical data set of 23 samples from St. Barthélemy Island, which belongs to the extinct branch of the Lesser Antilles arc and is currently exposed in the northern part of the subduction forearc. Samples were selected to represent all lithologies and main periods of magmatism, that is, Middle‐Late Eocene, Early Oligocene and Late Oligocene. They show enrichment in light rare earth element/medium rare earth element, large ion lithophile elements (Rb and Ba) and isotopic characteristics, suggesting mixing between the mantle and a subduction component (oceanic crust + sediments). Trace element ratios suggest that primary magmas were generated in a normal mid‐oceanic ridge basalt‐type mantle‐wedge that underwent 8%–18% partial melting in the spinel‐stability field. The sediment contribution was low (0.1%–1%) irrespective of the age of the samples. This is similar to what is observed for the northern Lesser Antilles active branch. St. Barthélemy Island shares strong similarities with St. Martin Island, located on the same extinct arc branch, which suggests a similar geodynamic evolution. Oligocene samples displayed an increase in incompatible elements in the magma source, suggesting an increase in sediment melts, which could be correlated with a drastic change in the tectonic regime at that time, characterized by stretching perpendicular to the trench and subsequent basin opening. On Δ7/4Pb versus 206Pb/204Pb, the samples showed a similar trend for both active and extinct islands of the northern Lesser Antilles, suggesting negligible changes in the nature of the magma sources. Plain Language Summary Subduction zones, and subduction‐related processes, are responsible for the recycling of oceanic crust and sediments deep down into the mantle, the formation of continental crust and ultimately have a profound influence on both climate and tectonics. Present‐day arcs formed above subduction zones are exceptional natural laboratories for studying arc‐related processes and testing models depicting the Earth's evolution through times. To understand subduction‐related processes, we collected 23 volcanic samples from St. Barthélemy Island (Lesser Antilles Arc, West French Indies) spanning from Eocene to late Oligocene. Geochemical analyzes (major and trace elements and Pb, Sr, Nd, Hf isotopes) can be used to constrain magma genesis at depth, the interactions between the various components involved and their variability over time. Analyzes show that the magmas were formed by 8%–18% partial melting of the mantle wedge with 0.1%–1% of sediment contribution. However, the Oligocene samples are characterized by increased sediment melting in the magma source, which we relate to a change in the tectonic regime and stretching perpendicular to the arc trench. A comparison with lavas from the Greater Antilles arc (Cuba) indicates sources and processes close to those active in the Lesser Antilles. Key Points Saint Barthélemy volcanics were formed by 8%–18% partial melting of an N‐MORB type mantle mixed with less than 1% of sediment melts Geochemical variation during the Early Oligocene is related to a sediment melt increase linked to a drastic change in the tectonic regime The extinct and active branches of the northern segment of the Lesser Antilles Arc display similar geochemical characteristics