Particle–polymer nanocomposites often exhibit mechanical properties described poorly bymicromechanical models that include only the particle and matrix phases. Existence ofan interfacial region between the particle and matrix, or interphase, has beenposited and indirectly demonstrated to account for this effect. Here, we present astraightforward analytical approach to estimate effective elastic properties ofcomposites comprising particles encapsulated by an interphase of finite thicknessand distinct elastic properties. This explicit solution can treat nanocompositesthat comprise either physically isolated nanoparticles or agglomerates of suchnanoparticles; the same framework can also treat physically isolated nanoparticleaggregates or agglomerates of such aggregates. We find that the predicted elasticmoduli agree with experiments for three types of particle–polymer nanocomposites,and that the predicted interphase thickness and stiffness of carbon black–rubbernanocomposites are consistent with measured values. Finally, we discuss the relativeinfluence of the particle–polymer interphase thickness and stiffness to identifymaximum possible changes in the macroscale elastic properties of such materials.