Purpose To develop and evaluate a semi-supervised learning model for intracranial hemorrhage detection and segmentation on an out-of-distribution head CT evaluation set. Materials and Methods This retrospective study used semi-supervised learning to bootstrap performance. An initial "teacher" deep learning model was trained on 457 pixel-labeled head CT scans collected from one U.S. institution from 2010 to 2017 and used to generate pseudo labels on a separate unlabeled corpus of 25 000 examinations from the Radiological Society of North America and American Society of Neuroradiology. A second "student" model was trained on this combined pixel- and pseudo-labeled dataset. Hyperparameter tuning was performed on a validation set of 93 scans. Testing for both classification ( = 481 examinations) and segmentation ( = 23 examinations, or 529 images) was performed on CQ500, a dataset of 481 scans performed in India, to evaluate out-of-distribution generalizability. The semi-supervised model was compared with a baseline model trained on only labeled data using area under the receiver operating characteristic curve, Dice similarity coefficient, and average precision metrics. Results The semi-supervised model achieved a statistically significant higher examination area under the receiver operating characteristic curve on CQ500 compared with the baseline (0.939 95% CI: 0.938, 0.940 vs 0.907 95% CI: 0.906, 0.908; = .009). It also achieved a higher Dice similarity coefficient (0.829 95% CI: 0.825, 0.833 vs 0.809 95% CI: 0.803, 0.812; = .012) and pixel average precision (0.848 95% CI: 0.843, 0.853) vs 0.828 95% CI: 0.817, 0.828) compared with the baseline. Conclusion The addition of unlabeled data in a semi-supervised learning framework demonstrates stronger generalizability potential for intracranial hemorrhage detection and segmentation compared with a supervised baseline. Semi-supervised Learning, Traumatic Brain Injury, CT, Machine Learning Published under a CC BY 4.0 license. See also the commentary by Swimburne in this issue.