Background Timely treatment of scapholunate instability depends on early identification, but current imaging methods are either intricate or fail to demonstrate the dynamic stages. Purpose To calculate the diagnostic accuracy of four-dimensional (4D) CT for diagnosing instable scapholunate ligament (SLL) tears. Materials and Methods This prospective study enrolled consecutive participants with clinically suspected SLL tears who underwent 4D CT from July 2020 to May 2022. A historical study sample diagnosed at cineradiography served as a comparison, and wrist arthroscopy was the reference standard. Scapholunate joints greater than 3 mm were interpreted as instable at index 4D CT and cineradiography. Diagnostic accuracy was expressed as sensitivity and specificity. Areas under the receiver operating characteristic curve and cutoff values for both index tests were calculated. Intraclass correlation coefficients (ICCs) were computed to compare interrater reliability. Effective radiation doses at 4D CT were measured with thermoluminescent dosimeters. Results The study included 40 participants (mean age, 43 years ± 14 [SD]; 24 male) evaluated at 4D CT and 78 patients (mean age, 45 years ± 11; 50 male) historically evaluated at cineradiography. Four-dimensional CT helped detect instable tears in 26 of 35 participants (sensitivity, 74.3% [95% CI: 56.7, 87.5]. Cineradiography revealed instable tears in 52 of 63 patients (sensitivity, 82.5% [95% CI: 70.9, 91]). Four of five participants with stable scapholunate joints were identified at 4D CT (specificity, 80.0% [95% CI: 28.4, 99.5]), and 12 of 15 patients with stable SLLs were identified at cineradiography (specificity, 80.0% [95% CI: 51.9, 95.7]). Interrater agreement of radiologic measurements on 4D CT scans was good to excellent (ICC range, 0.89-0.96). The effective radiation dose ranged from 67 to 72 mSv at the wrist and was less than 1 mSv at the head. Conclusion Four-dimensional CT results are highly reproducible. Instable scapholunate joints greater than 3 mm were detected with a sensitivity of 74.3% and a specificity of 80% in an exploratory trial. Further evidence from larger randomized trials is warranted. German Register for Clinical Trials no. DRKS00021110 (Universal Trial Number U1111-1249-7884) Published under a CC BY 4.0 license. Supplemental material is available for this article. See also the editorial by Demehri and Ibad in this issue.