Background: The albumin/globulin ratio (AGR) is one of several indicators of inflammation and immunity. This ratio has a prognostic significance in many malignant diseases. Previous studies have demonstrated a relationship between inflammatory mediators and post-operative pancreatic fistula (POPF). This study aimed to evaluate the relationship of AGR, a relatively new indicator, with post-operative pancreatic fistula (POPF). Methods: Pancreaticoduodenectomy (PD) patients between 2017 and 2020 were retrospectively analyzed and divided into two groups: (1) with and (2) without clinically relevant POPF (CR-POPF). The two groups were compared in terms of pre-operative–post-operative AGR and clinicodemographic characteristics. AGR was calculated as albumin/total protein–albumin, and the cutoff point for AGR was determined according to Youden’s index. Results: CR-POPF developed in 21% of 121 patients who underwent PD. No differences between the groups in terms of age, gender, comorbid disease status, pancreatic duct width, and anastomosis technique were found. Pre- and post-operative day-3 (POD3) albumin levels and AGR were found to be significantly lower in the CR-POPF group. Multivariate analysis showed that AGR and pancreatic tissue stiffness are independent risk factors for POPF development. Conclusion: Low AGR is an independent risk factor for the development of CR-POPF. To reduce the incidence of POPF, this ratio should be maintained at an optimal level. The use of AGR as a useful tool for predicting POPF in pancreaticoduodenectomy patients is suggested.