ObjectivePostoperative delirium (POD) is a common complication along with poor prognosis in geriatric intertrochanteric fracture (ITF) patients. However, the prevention and treatment of POD remain unclear. Previous studies have confirmed that POD is essentially a consequence of neuro-inflammatory responses. Dexamethasone is a glucocorticoid with comprehensive anti-inflammatory effects, while a high dose of dexamethasone correlates with many side effects or even adverse consequences. Thus, this prospective study aims to discuss whether a single preoperative low-dose dexamethasone can reduce the impact of POD on geriatric ITF patients with internal fixation surgery.MethodsBetween June 2020 and October 2022, there were 219 consecutive ITF patients assessed in our department. Of the 219 ITF patients, 160 cases who met the inclusion and exclusion criteria were finally enrolled and randomly allocated to the dexamethasone group and the placebo group (80 geriatric ITF patients in each group) in this prospective study. The patients in the dexamethasone group received intravenous 10 mg (2 ml) dexamethasone while the patients in the placebo group received intravenous 2 ml saline in 30 min before being sent to the operating room, respectively. The baseline characteristics, surgical information, incidence and severity of POD as the efficacy-related outcomes, and infection events and hyperglycemia as safety-related outcomes (adverse events), were collected and analyzed between the two groups. The severity of POD was evaluated by Memorial Delirium Assessment Scale (MDAS) score.ResultsThere were no differences in baseline characteristics and surgical information between the dexamethasone group and the placebo group. The dexamethasone group had a lower incidence of POD than the placebo group within the first 5 days after surgery [(9/80, 11.3% vs. 21/80, 26.3%, RR = 0.83, 95% CI 0.71–0.97,P= 0.015]. The dexamethasone group had lower MDAS scores (Mean ± SD) than the placebo group [13.2 ± 1.0 (range 11 to 15) vs. 15.48 ± 2.9 (range 9 to 20),P= 0.011, effect size = 0.514]. There were no differences in infection events and hyperglycemia between the two groups.ConclusionsA single preoperative low-dose dexamethasone may reduce the incidence and severity of POD in geriatric ITF patients with internal fixation surgery.Trial registration: ChiCTR2200055281.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13018-023-03930-2.