The ferroic phase transition in LaEr(MoO₄)₃ has been analyzed for the first time. It has been confirmed that this compound undergoes a phase transition from a tetragonal system (paraelectric-paraelastic phase), with space group P-42₁m [β-Gd₂(MoO₄)₃ averaged phase] to an orthorhombic system (ferroelectric-ferroelastic phase), with space group Pba2 [β'-Gd₂(MoO₄)₃ phase] in a reversible process. This phenomenon, together with the observed demixing at high temperature has been studied using different techniques. LaEr(MoO₄)₃ samples have been obtained by the conventional solid-state synthesis. The thermal dependence of the crystal structure was studied by powder X-ray and neutron diffraction, following a new refining procedure in which the symmetry modes of atomic displacements from the paraelectric-paraelastic structure were analyzed. Dielectric spectroscopy measurements have confirmed the structural results, showing a very smooth phase transition. Finally, calculations within the framework of Density Functional Theory show a behavior of the lattice parameters similar to that observed in our experiments.