XtalOpt is an open-source code, published under the “NEW” BSD License, designed to perform multi-objective evolutionary search for novel functional materials. Local optimizations can be performed using a variety of explicitly supported first-principles codes and classical force fields (e.g. VASP, Quantum-Espresso, Abinit, and GULP); while any desired total energy calculation tool (e.g., machine learning potentials) can be utilized through scripting.