Introduction Robotic prostatectomy requires pneumoperitoneum and a steep Trendelenburg position; however, this condition may compromise cerebral blood flow. Here, we evaluated the effect of pneumoperitoneum and the steep Trendelenburg position on internal carotid artery (ICA) blood flow measured by Doppler ultrasound during robotic prostatectomy. Methods Patients who underwent robotic prostatectomy were prospectively recruited. The ICA blood flow was measured at the following five time‐points: with the patient awake and in the supine position (Ta), 10 min after anaesthetic induction in the supine position (T1), 10 (T2) and 30 (T3) min after pneumoperitoneum in the steep Trendelenburg position, and at the end of surgery in the supine position after desufflation of the pneumoperitoneum (T4). Hemodynamic and cerebrovascular variables were measured at each time‐point. Results A total of 28 patients were evaluated. The ICA blood flows were significantly lower at T2 and T3 than at T1 (162.3 ± 44.7 [T2] vs. 188.0 ± 49.6 ml/min [T1]; p = .002, 163.1 ± 39.9 [T3] vs. 188.0 ± 49.6 ml/min [T1]; p = .009). The ICA blood flow also differed significantly between Ta and T1 (236.8 ± 58.3 vs. 188.0 ± 49.6 ml/min; p < .001). Heart rates, cardiac indexes, peak systolic velocity, and end‐diastolic velocity were significantly lower at T2 and T3 than at T1. However, ICA diameter, mean blood pressure, and end‐tidal carbon dioxide partial pressure did not differ significantly at all time‐points. Conclusion Pneumoperitoneum and the steep Trendelenburg position caused decreased ICA blood flow, suggesting that they should be carefully performed during robotic prostatectomy, especially in patients at risk of postoperative cerebrovascular accident.