Background/Objectives: Endoscopic ultrasound-guided fine-needle biopsy (EUS-FNB) is a useful procedure for obtaining histological specimens. However, its utility in diagnosing autoimmune pancreatitis (AIP) has not yet been well studied. This study aimed to assess the diagnostic capability of EUS-FNB for AIP by comparing a 19-gauge Franseen needle (19FR) and a 22-gauge Franseen needle (22FR). Methods: This study included patients with a final diagnosis of AIP undergoing EUS-FNB for pancreatic lesions between January 2014 and February 2023. All patients underwent EUS-FNB with either 19FR or 22FR. Histological findings were evaluated according to the International Consensus Diagnostic Criteria (ICDC). The primary outcome was the diagnostic yield of Level 1 (≥3 ICDC items) or Level 2 (2 ICDC items). Results: The 19FR group included 31 patients, and the 22FR group included 36 patients. The Level 1 diagnostic rate was significantly higher in the 19FR group than in the 22FR group (90.3% vs. 61.1%, p = 0.010). No significant difference was observed in the Level 2 diagnostic rate. The 19FR group yielded significantly larger histological tissue samples than the 22FR group (median area: 9.19 mm²/session vs. 3.36 mm²/session, p < 0.001). The analysis demonstrated a positive correlation between tissue area and the number of histological diagnostic items obtained. Conclusions: EUS-FNB performed with the 19FR provided larger histological specimens and a higher histological diagnostic yield than the 22FR in the diagnosis of AIP. Obtaining a larger amount of tissue may facilitate a definitive diagnosis of AIP.