Background: In robot-assisted laparoscopic prostatectomy (RALP), restrictive fluid therapy (RFT) is often utilized until the vesicourethral anastomosis (console period) is completed. RFT can cause acute kidney injury (AKI). Thus, RFT prolongation in surgeries that utilize the Trendelenburg position and pneumoperitoneum may increase the risk of postoperative AKI. We aimed to evaluate the effect of RFT duration on postoperative AKI. Methods: Forty-four patients who underwent RALP were included in this prospective observational study. Patients were divided into two groups according to the RFT duration (Group I, RFT duration ≤ 3 h, and Group II, RFT duration >3 h). AKI was diagnosed and staged according to the Kidney Disease Improving Global Outcomes criteria (KDIGO) using patients’ serum creatinine levels after the first 24 h postoperatively. Hemodynamic parameters were monitored using the pressure recording analytical method. Results: The AKI incidence was significantly higher in Group II than in Group I (45.5% vs. 9.1%; p = 0.016). In both groups, all patients who developed AKI were KDIGO stage 1 and all recovered on the second postoperative day. At the end of the console period, the heart rate and arterial elastance were significantly higher, whereas the stroke volume index was significantly lower in Group II than in Group I (p = 0.041, p = 0.016, and p < 0.001, respectively). Although the amounts of fluid administered before and after the anastomosis were similar between the groups, the total amount of fluid administered was significantly different (p < 0.001). There was a significant negative correlation between RFT duration and the total amount of fluid administered (r2 = 0.43, p < 0.001). RFT duration of >3 h, total fluid administration of ≤3.3 mL/kg/h, and stroke volume index (SVI) at the end of the console period of ≤32 mL/m2 increased the risk of AKI by 12.0 times (1.7–85.2) (p = 0.013). Conclusion: RFT prolongation in RALP may increase the risk of developing AKI.