The processability of organic thermoelectric materials plays a crucial role due to their clear advantages of applicability in large-scale areas compared to traditional inorganic counterparts. A promising way to process thermoelectric materials based on conductive polymers is through in situ polymerization in an insulating polymer matrix. This work shows an interpenetrating polymeric network based on polythiophene, silver nanoparticles (Ag NPs), and poly(methyl methacrylate) (PMMA) produced by the oxidative polymerization of terthiophene by an oxidizing silver salt in a PMMA matrix. Ag NPs are in situ synthesized simultaneously as a byproduct. The reaction occurs very fast in the solid state, and after only 1 min, a homogeneous interpenetrating polymer network (IPN) film is obtained, reaching electrical conductivity values of 120 S cm–1. Ag NPs play a determining role in the conducting properties of the IPN. Moreover, the thermoelectric properties were evaluated as a function of the synthesis parameters, reaching a maximum power factor of 51 μW m–1 K–2. This study shows a promising method to enhance the processability of hybrid thermoelectric materials on the basis of conductive polymers and nanofillers.