EID1 (E1A-like inhibitor of differentiation 1) functions as an inhibitor of nuclear receptor-dependent gene transcription by directly binding to co-regulators. Alternative targets include the co-repressor small heterodimer partner (SHP, NR0B2) and the co-activators CBP/p300, indicating that EID1 utilizes different inhibitory strategies. Recently, EID2 was characterized as an inhibitor of muscle differentiation and as an antagonist of both CBP/p300 and HDACs. Here, we describe a third family member designated EID3 that is highly expressed in testis and shows homology to a region of EID1 implicated in binding to CBP/p300. We demonstrate that EID3 acts as a potent inhibitor of nuclear receptor transcriptional activity by a mechanism that is independent of direct interactions with nuclear receptors, including SHP. Furthermore, EID3 directly binds to and blocks the SRC-1 interacting domain of CBP, which has been implicated to act as the interaction surface for nuclear receptor co-activators. Consistent with this idea, EID3 prevents recruitment of CBP to a natural nuclear receptor-regulated promoter. Our study suggests that EID-family members EID3 and EID1 act as inhibitors of CBP/p300-dependent transcription in a tissue-specific manner.