phbuilder is a Python-based command line tool that assists in setting up and performing constant pH MD simulations with GROMACS. Both the tool and a user manual (including tutorials) are available at www.gitlab.com/gromacs-constantph/. To prepare constant pH MD simulations with phbuilder, the only required input is a structure file (.pdb or .gro) and the desired simulation pH. The structure can be an initial protein structure or a prepared simulation system. During the setup process, the user can optionally provide additional parameters such as the desired ion concentration and the number of buffer particles to add.