A bulk photovoltaic effect (BPVE) in materials without inversion symmetry attracts increasing interest for high-efficiency solar cells beyond the p-n junction paradigm. Herein, we report the photovoltaic effect in an experimentally feasible TlNbXO monolayer (TlNbXO-ML, X = Cl, Br, I) with a large ferroelectric polarization. Using first-principles calculations, we demonstrate that TlNbXO-MLs are ferroelectric semiconductors with moderate switching barriers and higher spontaneous polarizations. Furthermore, we observe fairly giant shift current with the values of 109.6 μA/V for TlNbClO, 60 μA/V for TlNbBrO, and 56.1 μA/V for TlNbIO. These results unveil distinct features of the BPVE and the potential application of two-dimensional ferroelectric materials for next-generation photovoltaic devices.