We present a radial velocity study of the rapidly rotating B star Regulus that indicates the star is a single-lined spectroscopic binary. The orbital period (40.11 days) and probable semimajor axis (0.35 AU) are large enough that the system is not interacting at present. However, the mass function suggests that the secondary has a low mass (M2 > 0.30 M⊙), and we argue that the companion may be a white dwarf. Such a star would be the remnant of a former mass donor that was the source of the large spin angular momentum of Regulus itself. © 2008. The American Astronomical Society. All rights reserved.