Hygrothermal conditions significantly affect the mechanisms underlying damage in polymer nanocomposites [1, 2]. Here, we develop a phase-field formulation to study epoxy nanocomposites’ temperature, moisture, and rate-dependent fracture behavior at finite deformation. The Helmholtz free energy is defined based on an additive decomposition of the energy into an equilibrium, a non-equilibrium, and a volumetric contribution with different definitions under tensile and compressive loading. The coupled displacement phase-field problem is solved using a monolithic solver based on a fracture energy-based arc-length method. The monolithic solver enables overcoming snap-back and snap-through instabilities during specimen loading. The effect of temperature, deformation rate, and moisture content on the force-displacement response of boehmite nanoparticle/epoxy samples are investigated. Comparing numerical predictions with experimental data for compact-tension tests shows good agreement at different nanoparticle contents.