The multilayer multiconfiguration time-dependent Hartree theory is applied to study the quantum dynamics of ultrafast electron-transfer reactions in a condensed-phase environment with anharmonic potential functions. Effects of anharmonicity for both the nuclear degrees of freedom of the environment and the intramolecular vibrational degrees of freedom are investigated. Whereas the former can in principle be mapped to a fictitious harmonic bath, the latter cannot be represented in this way and, thus, go beyond the commonly employed linear response approximation. Numerical examples are presented to illustrate these findings.