Weak localization was observed in a black phosphorus field-effect transistor 65 nm thick. The weak localization behavior was found to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up to 1 T, from which characteristic scattering lengths could be inferred. The temperature dependence of the phase coherence length Lφ was investigated, and above 1 K, it was found to decrease weaker than the LφT-1/2 dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in quasi-one-dimensional systems such as metallic nanowires and carbon nanotubes.