Background and Purpose: We aimed to determine the prevalence and predictors of delayed neurological improvement (DNI) after complete endovascular reperfusion in anterior circulation acute ischemic stroke (AIS). Methods: Retrospective analysis of an online multicenter prospective reperfusion registry of patients with consecutive anterior circulation AIS treated with endovascular thrombectomy (EVT) from January 2018 to June 2019 in tertiary stroke centers of the NORDICTUS (NORD-Spain Network for Research and Innovation in ICTUS) network. We included patients with AIS with a proximal occlusion in whom a modified Thrombolysis in Cerebral Infarction 3 reperfusion pattern was obtained. DNI was defined if, despite absence of early neurological improvement during the first 24 hours, patients achieved functional independence on day 90. Clinical and radiological variables obtained before EVT were analyzed as potential predictors of DNI. Results: Of 1565 patients with consecutive AIS treated with EVT, 1381 had proximal anterior circulation occlusions, 803 (58%) of whom achieved a modified Thrombolysis in Cerebral Infarction 3. Of these, 628 patients fulfilled all selection criteria and were included in the study. Mean age was 73.8 years, 323 (51.4%) were female, and median baseline National Institutes of Health Stroke Scale was 16. Absence of early neurological improvement was observed in 142 (22.6%) patients; 32 of these (22.5%) achieved good long-term outcome and constitute the DNI group. Predictors of DNI in multivariable-adjusted logistic regression were male sex (odds ratio, 6.4 [95% CI, 2.1–22.3] P =0.002), lower pre-EVT National Institutes of Health Stroke Scale score (odds ratio, 1.4 [95% CI, 1.2–1.5], P <0.001), and intravenous thrombolysis (odds ratio, 9.1 [95% CI, 2.7–30.90], P <0.001). Conclusions: One-quarter of patients with anterior circulation AIS who do not clinically improve within the first 24 hours after complete cerebral endovascular recanalization will achieve long-term functional independence, regardless of the poor early clinical course. Male sex, lower initial clinical severity, and use of intravenous thrombolysis before EVT predicted this clinical pattern.