The androgen receptor (AR) has two transactivation functions that have been mapped to the N- and C-terminal domains and designated as activation function-1 (AF-1) and AF-2, respectively. While the molecular basis for AF-2 function has been well studied, little is known about AF-1 coregulators. Therefore, we attempted to identify AF-1-interacting proteins from HEK293 cells by biochemical purification followed by mass fingerprinting by matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI–TOF MS). Purified AF-1 region-interacting proteins were found to contain nuclear RNA-binding protein p54^nrb, polypyrimidine tract-binding protein-associated splicing factor (PSF), paraspeckle protein 1 (PSP1), and PSP2, which are assumed to be involved in pre-mRNA processing. p54^nrb interacted with AR via the A/B domain in a ligand-dependent manner. Reflecting the physical interaction between p54^nrb and the AR A/B domain, AR AF-1 function was potentiated by p54^nrb. Our results suggest that p54^nrb functions as a coactivator of AR that potentiates transcription, and presumably splicing as well.