Background/Objectives: This study evaluated the diagnostic performance of serum leucine-rich alpha-2-glycoprotein 1 (LRG1) and plasma calprotectin in adults with suspected acute appendicitis, compared them with routine inflammatory markers and clinical scores, and examined their association with abdominal guarding and the discrimination of uncomplicated from complicated appendicitis. Methods: Adults aged ≥18 years were enrolled in two Croatian emergency departments from April 2025 to March 2026. Acute appendicitis was histopathologically confirmed, whereas controls were defined by clinical follow-up or negative appendiceal histopathology. Biomarkers were measured using enzyme-linked immunosorbent assays, and diagnostic performance was assessed using receiver operating characteristic (ROC) analysis with area under the curve (AUC). Results: Among 126 patients, 62 (49.2%) had acute appendicitis and 64 (50.8%) were controls. Serum LRG1 was higher in patients with acute appendicitis than controls (27.4 μg/mL; IQR 25.3–28.0 vs. 23.2 μg/mL; IQR 17.6–27.8; p = 0.01), whereas plasma calprotectin did not differ. LRG1 was higher in cases with abdominal guarding (p = 0.02), but neither biomarker differed significantly between uncomplicated and complicated appendicitis. Conclusions: LRG1 showed limited diagnostic accuracy, while calprotectin showed no meaningful discrimination. The Adult Appendicitis Score and Appendicitis Inflammatory Response score outperformed both biomarkers.