Entangled photon pairs are a key resource in future quantum-optical communication and information technologies. While high-power laser light propagating in bulk nonlinear optical crystals is conventionally used to generate entangled photons that are routed into optical configurations, such schemes suffer from low efficiency due to the weak intrinsic nonlinear optical response of known materials and losses associated with photon in- and out-coupling. Here, we propose a scheme to generate entangled polariton pairs directly within optical waveguides using free electrons, whereby the measured energy loss of undeflected electrons heralds the production of counter-propagating polaritons pairs that are entangled in energy and direction of emission. As a paradigmatic example, we study the excitation of plasmon polaritons in metal strip waveguides that, within specific frequency regimes, strongly enhance light-matter interactions that lead to two-plasmon generation in comparison to the probability of single-plasmon excitation. We demonstrate that, under appropriate conditions, an electron energy loss detected in an optimal frequency range can reliably signal the generation of a plasmon pair entangled in energy and momentum. Our proposed scheme can be directly applied to other types of optical waveguides for in situ generation of entangled photon pairs in quantum-optics applications.