A high-performance microwave absorber was created using Fused Deposition ModelingModeling (FDM) 3D printing3D printing. Both a narrow band and a broadband absorber were created. The narrow band absorber was designed at 4.9 GHz, mid-band in WR-187 waveguideWaveguide. The broadband absorber tried to achieve the best attenuationAttenuation across the entire 3.95–5.85 GHz band. Two types of carbon loaded polylactic acid (PLA) plastic and unloaded PLA were printed with variable percentages of air to create different values of dielectricDielectrics constant and loss tangent. A stack of five or six rectangular pieces of plastic each with different characteristics were optimized through computer simulation in HFSS and analytic modelingModeling in MATLAB to maximize attenuationAttenuation. The stack progressed from the lowest loss and lowest dielectricDielectrics constant to the highest at the shorting end. The final narrow band load had simulated return loss of 291 dB at 4.9 GHz with an analytic solution in MATLAB, and a corresponding result of 119 dB in HFSS. The measured return loss of the 3D printed attenuator was 73.254 dB at 4.929 GHz. The total length of the absorber is 2.44 inches. A commercial absorber for WR-187 with a return loss of 40 dB has a length of 13 inches. The experiment proves that an effective microwave absorber can be created using 3D printing3D printing.