Spin-orbit torque (SOT) has been studied extensively in a heavy-metal (HM)/ferromagnet (FM) bilayer structure, where a HM is an essential ingredient because a spin current is generated via the spin Hall effect within the HM layer and/or the Rashba-Edelstein effect from the HM/FM interface. Here, we report the observation of SOT in a ferrimagnetic $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer with perpendicular magnetic anisotropy without a HM layer. Using harmonic Hall voltage measurements, we investigate the SOT-induced dampinglike effective field $({B}_{\mathrm{DL}})$ in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer; the sign of ${B}_{\mathrm{DL}}$ is opposite from that of a $\mathrm{Pt}/\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ bilayer, and the magnitude of ${B}_{\mathrm{DL}}$ increases as the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ composition approaches its magnetization compensation point. Moreover, we analyze the elemental composition of $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ as a function of film thickness using scanning transmission electron microscopy and electron energy-loss spectroscopy, indicating that the sign and magnitude of the SOT are virtually insensitive to the vertical composition gradient within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer. Our results demonstrate that the bulk spin-orbit interaction within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer itself plays a major role in generating SOT in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer.
