In this investigation, the influence of section diameter on high cycle fatigue (HCF) behavior of additively manufactured 316L stainless steel was characterized. Three gauge-section diameters (5.0 mm, 2.5 mm, and 1.5 mm) were examined for their influence on the ultrasonic fatigue response of samples built via laser-powder bed fusion (L-PBF). HCF was conducted under full reversed loading (R=-1) conditions. A total of 130 specimens were characterized in the as-built state at maximum stresses ranging from 70 to 220 MPa. A Random Fatigue Limit (RFL) model using a Maximum Likelihood Estimation (MLE) was used to quantify statistical variability and estimate an S-N curve fit. The fatigue response shows that the largest gauge diameter (5.0 mm) resulted in the lowest fatigue strength at 89.5 ± 5.6 MPa, and the smallest diameter (1.5 mm) resulted in the highest fatigue strength at 122.0 ± 32.8 MPa. The 2.5 mm diameter specimens exhibited a fatigue strength of 98.7 ± 7.0 MPa. The primary failure mechanism in all as-built specimens was surface initiated cracking from crevices in the as-built surface finish. Additional specimens with a nominal diameter of 5.0 mm were fatigue tested with the as-built surface removed via low stress surface grinding. The fatigue strength of these samples increased to 170 MPa when 75 m of the surface was removed and 179 MPa when the surface contour was entirely removed. Residual stresses were characterized by x-ray diffraction (XRD) and show a reduced axial residual stress with reduction in gauge diameter. Additional specimens were fatigue tested after undergoing a stress relief anneal, resulting in a 51% reduction in the residual stress and a 30% improvement in fatigue strength. An in-depth analysis of the microstructure, surface roughness, defects, and fracture surface indicate that both the surface condition and residual stress are the primary factors influencing the observed diameter effects on HCF.