Xyleborus vochysiae n. sp. (Curculionidae: Scolytinae) is described and illustrated. This newly discovered ambrosia beetle was collected from live trees of Vochysia ferruginea Mart. in wet tropical forests of Costa Rica. The new species does not fit readily into either Ambrosiodmus or Xyleborus, but it is placed in the latter genus on the balance of the evidence; it is not obviously close to any Ambrosiodmus or Xyleborus in Central America. X. vochysiae is extraordinary in three regards: it is the largest Xyleborini found in the Neotropics, it has only been collected from live trees, and it seems to be restricted to only one host species. On a world basis, only five other ambrosia beetles are known that regularly breed in live trees, and no species are known that are as host specific as is X. vochysiae.