The wave infill for material extrusion (MEX) of the thin-walled structure (TWS) is presented. The wave infill, a lightweight truss-like porous core structure sandwiched between two outer walls, is an efficient toolpath pattern for the MEX of TWS. Analytical models for predicting the stiffness, load capacity, fabrication time, and mass were established for two orthogonal in-plane and layer-to-layer variations inherent in MEX wave infill parts. Rectangular prism, four-point flexural bending specimens representing the in-plane and layer-to-layer orientations with wave infill were fabricated by MEX of Polyamide-12 (Nylon-12) material. From these specimens, fabrication time and mass were measured, and four-point flexural tests were conducted to measure the stiffness and load capacity of the beam. Analytical models were compared with the experimental measurements to identify their predictive capabilities. Stiffness for in-plane and layer-to-layer orientations was predicted well with the relative root-mean-square error (RRMSE) of 7% and 6%, respectively. Load capacity in in-plane and layer-to-layer orientations had the RRMSE of 23 and 22%, respectively. Fabrication time and mass were predicted well with a RRMSE of 7 and 6%, respectively. The methods established in this study are the foundation for optimal design and MEX of wave infill TWSs with generalized loads.