Background: Delirium is a common complication after proximal femoral fracture surgery, with pain and opioid consumption as the contributing factors. The administration of intrathecal morphine may decrease these factors postoperatively and potentially reduce delirium. Objectives: This research aimed to study the association between the use of intrathecal morphine and the occurrence of delirium. Methods: A retrospective analysis of a prospective register kept in a non-academic hospital in the Netherlands was performed. The register contained data of all patients with proximal femur fractures that were surgically treated with osteosynthesis or prosthesis. Patients receiving spinal anesthesia (SA group) were compared with patients receiving spinal anesthesia with the addition of intrathecal morphine (SIM group). The administration of either SA or SIM was based on the preference of the anesthesiologist. The primary outcome was the incidence of delirium, as defined by the DSM-V classification. The follow-up lasted until hospital discharge. Both univariate and multivariate analyses were performed. Results: The SA group consisted of 451 patients, and the SIM group included 34 patients. Delirium occurred in 19.7% in the SA group versus 5.9% in the SIM group (P = 0.046). This association remained significant after correction in multivariate analysis (OR of delirium in the SA group, 95% CI: 1.062 - 21.006, P = 0.041). Additionally, multivariate analysis revealed that age, gender, preoperative cognitive impairment, and fracture treatment (osteosynthesis or prosthesis) were independently associated with delirium. Conclusions: This retrospective study found an independent association between the use of intrathecal morphine and a lower incidence of delirium. This clinically relevant decrease in delirium should be studied in a prospective randomized study.