Purpose: This prospective, single-center cohort study analyzes the potential of inflammatory protein mediator leucine-rich alpha-2 glycoprotein 1 (LRG1) for the early and accurate diagnosis of acute appendicitis (AA), and differentiation of acute complicated (AcA) from uncomplicated appendicitis (AuA). Methods: Participants were divided into the AcA, AuA, and control groups, and their serum (s-LRG1) and urine LRG1 (u-LRG1) levels were assayed preoperatively on the second and fifth postoperative days. Results: 153 patients participated, 97 had AA. Preoperative u-LRG1 with a cut-off value of 0.18 μg/mL generated an area under the receiver operated characteristic (AUC) curve of 0.70 (95% CI 0.62–0.79) for AA versus control (p < 0.001), while the results for AcA versus AuA were not significant (AUC 0.60, 95% CI 0.49–0.71, p = 0.089). The s-LRG1 levels of AA versus the control with a cut-off value of 51.69 μg/mL generated an AUC of 0.94 (95% CI 0.91–0.99, p < 0.001). The cut-off value of s-LRG1 was 84.06 μg/mL for diagnosis of AcA from AuA, and therefore, significant (AUC 0.69, 95% CI 0.59–0.80, p = 0.001). Conclusions: LRG1 exhibited excellent diagnostic performance as an inexpensive, non-invasive, rapid, and accurate biomarker able to reflect the pathogenesis of AA. LRG1 has the potential to replace advanced imaging to diagnose clinically ambiguous AA cases.