Continuous measurement of flap tissue perfusion in the context of postoperative flap monitoring after microvascular head and neck reconstruction may be confounded by alterations in systemic blood pressure, particularly when using predefined absolute thresholds for the detection of vascular flap compromise. This study aimed to investigate the relationship between radial free forearm flap (RFFF) tissue perfusion and systemic blood pressure following head and neck reconstruction. The study included 14 patients reconstructed with an RFFF in the head and neck region between 2020 and 2022. Eight hours of postoperatively recorded flap tissue perfusion, determined with an attached surface probe at a 3 mm tissue depth with the Oxygen-2-See (O2C) analysis system, in terms of blood flow and hemoglobin oxygen saturation, and systemic blood pressure, determined as absolute and relative values (difference between measured and preoperative values), in terms of systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial blood pressure (MBP), were analyzed for correlations. Blood flow and hemoglobin oxygen saturation were generally indifferent between the hourly measurement intervals. Blood flow was correlated with relative DBP and MBP values (0.191, < 0.001; and 0.213, < 0.001). These correlations persisted upon controlling for norepinephrine, propofol, or sulfentanyl (all < 0.001). Early postoperative RFFF tissue perfusion measured with attached surface probes for the O2C analysis system remains constant but correlates with systemic blood pressure in terms of blood flow and relative blood pressure values. This highlights the importance of maintaining constant systemic blood pressure during RFFF tissue perfusion measurement for postoperative flap monitoring.