The interactions between an aliovalent cation dopant, Ba, and protons in LaPO₄ are studied with first-principles density functional theory. This work is motivated by the desire to use doped LaPO₄ as a proton conducting solid electrolyte or hydrogen separation membrane. In this context, the strength and range of proton-dopant interactions are important factors underlying proton mobilities. Using periodic supercells, we find that ∼3% Ba-doping stabilizes a proton at a distance 2.7 Å from the dopant by up to 0.2 eV relative to positions far from the dopant. The Ba-dopant creates a narrow potential energy well and only changes the proton's potential energy surface by ±0.05 eV when the proton is farther than 2.7 Å from the dopant. Electrostatic interactions between the proton and dopant account for the majority of the binding energy of proton sites and are associated with a complex redistribution of the charge induced by the dopant on the neighboring oxygen ions. In contrast, the strain field created by the Ba-dopant gives rise to a relatively small contribution to the interaction energy.