Surgical site infection (SSI) is one of the most common postoperative complications in patients undergoing major operations, such as spinal fusion surgery, and a major contributor to patient morbidity and mortality. SSI is considered the most preventable type of infection; however, the risk of SSI is multifactorial. This study aimed to determine the extent to which the expertise of the surgical first assistant (SFA) affected SSI rates. We retrospectively reviewed 528 patients at a single institution who underwent lumbar spine fusion surgery via the posterior approach performed by a single surgeon between January 2012 and May 2020. The SFAs participating in the surgeries were classified into 2 groups: a certified neurosurgery specialist and relatively less experienced neurosurgery resident trainees. To reduce potential selection bias and confounding factors, propensity score matching was performed between the 2 groups. In 170 of the 528 lumbar spine fusion surgeries, the SFA was a certified neurosurgery specialist. In the other 358 surgeries, the SFA was a resident trainee. Seventeen patients met the SSI criteria. The SSI rate was significantly different between the 2 groups (0.6% (1 patient) and 4.5% (16 patients) in the certified specialist and resident trainee groups, respectively; P = .02). After propensity score matching, 170 paired patients were selected. After adjusting for confounding factors, SFAs that were certified neurosurgery specialists were associated with a lower likelihood of SSI (adjusted OR 0.09; 95% CI, 0.01 to 0.79; P = .029) than SFAs that were neurosurgery residents. A higher level of SFA expertise was significantly associated with a lower overall SSI rate in lumbar spine fusion surgeries. It is difficult to predict the incidence of SSI; however, this finding suggests the importance of SFA expertise in preventing SSI.