Aim: The purpose of this study was to evaluate the diagnostic value of ultrasound compared to magnetic resonance imaging (MRI) as a reference in detecting peroneus brevis split ruptures. Material and methods: We re-reviewed 112 ultrasound examinations performed between 2020 and 2021 by three musculoskeletal radiologists with 8–10 years of experience. Patients were referred due to pain lasting at least 8 months in the posterolateral ankle. Ultrasound was performed using a LOGIQ E9 General Electric device with a 6–15 MHz or 18 MHz probe. Sixty-three patients who underwent MRI within 8 months and were included in the study. Ultrasound and MRI findings were categorized as: a) no peroneus split, b) presence of peroneus split, or c) unspecific findings. MRI served as the reference standard. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were calculated. Results: Seven cases (11.1%) were false positives (diagnosed on ultrasound but not MRI) and 9 (14.3%) were false negatives (missed by ultrasound but detected on MRI). Six cases (9.5%) were true positives (identified on both ultrasound and MRI), and 41 patients (65.1%) were true negatives (negative on both modalities). Ultrasound showed a sensitivity of 40.0% and specificity of 85.4%. The positive predictive value (PPV) was 46.2%, while the negative predictive value (NPV) was 82.0%. Conclusions: Ultrasound demonstrated limited sensitivity but high specificity in detecting peroneus brevis split ruptures.