Background Recent evidence suggests that presence of an intracranial arterial thrombus with a hyperdense artery sign (HAS) at noncontrast CT (NCCT) is associated with better response to intravenous alteplase. Patients with HAS may benefit more from combined intravenous alteplase and endovascular treatment (EVT). Purpose To investigate whether HAS at NCCT modifies the treatment effect of adding intravenous alteplase on clinical outcome in patients with acute large-vessel occlusion undergoing EVT. Materials and Methods This study is a secondary analysis of a prospective randomized trial (Direct Intra-arterial thrombectomy in order to Revascularize AIS patients with large-vessel occlusion Efficiently in Chinese Tertiary hospitals: A Multicenter randomized clinical Trial [DIRECT-MT]), which compared adding alteplase to EVT versus EVT alone in participants with acute large-vessel occlusion between February 2018 and July 2019. Participants with catheter angiograms and adequate NCCT for HAS evaluation were included. HAS was determined visually by two independent investigators at baseline NCCT. Treatment effect of intravenous alteplase administration according to presence of HAS on the primary clinical outcome (modified Rankin Scale [mRS] score at 90 days) and secondary and safety outcomes were assessed using adjusted multivariable regression models. Results Among 633 included participants (356 men [56%]; median age, 69 years), HAS was observed in 283 participants (45%): 142 of 313 participants (45%) in the EVT-only group and 141 of 320 participants (44%) in the group with added intravenous alteplase. Treatment-by-HAS interaction was observed for the primary outcome (P < .001), whereby a shift in favor of better outcomes with added intravenous alteplase occurred in participants with HAS (adjusted odds ratio [OR]: 1.82; 95% CI: 1.18, 2.79), while an adverse effect was seen in participants without HAS (adjusted OR: 0.62; 95% CI: 0.42, 0.91). This also held true for three secondary outcomes (excellent outcome [mRS score of 0-1 at 90 days], P = .005; good outcome [mRS score of 0-2 at 90 days], P = .008; final successful reperfusion, P = .04) in the adjusted models. Conclusion After acute ischemic stroke, presence of hyperdense artery sign (HAS) at baseline noncontrast CT indicated better outcomes when alteplase was added to endovascular treatment, but adding alteplase to endovascular treatment resulted in worse outcomes in participants without HAS. Clinical trial registration no. NCT03469206 © RSNA, 2022 Online supplemental material is available for this article.