Polyether ether ketone (PEEK) is a thermoplastic polymer renowned for its exceptional thermal resistance, mechanical strength, durability, chemical resistance, and low density. PEEK can be additively manufactured using various techniques, including Power Bed Fusion (PBF) and Material Extrusion (MEX). Among these, MEX stands out for its ease of operation and cost-effectiveness. Optimizing printing parameters and thermal post-processing are crucial for enhancing the mechanical and thermal properties of printed parts. In this study, the effect of an infiltration process by a high-performance thermosetting epoxy resin on the flexural properties of PEEK has been investigated. Moreover, a comparison with respect to as-printed specimens has been carried out. Results revealed that infiltrated PEEK exhibits a 27,4% higher flexural strength, with an average value of 93,84 MPa, and a 12,5% higher flexural modulus, averaging at 3311,44 MPa. Furthermore, the strength-to-weight ratio and stiffness-to-weight ratio both experienced improvements in specimens subjected to infiltration. Notably, as-printed PEEK displayed increased susceptibility to delamination under flexural stress, demonstrating a more brittle fracture behavior compared to infiltrated specimens. In summary, the findings indicate that the developed infiltration post-processing method shows potential for improving the mechanical properties of additively manufactured PEEK.