Abstract Background Acute injuries to the ulnar collateral ligament (UCL) of the first metacarpophalangeal joint (MCPj), commonly referred to as Skier’s thumb injuries, can be further categorized into different types based on bone lesions and joint instability. However, none of the existing classifications encompasses all types and combinations of injuries. A unified classification system that provides direct treatment recommendations for each type is required, particularly for cases with no clear indications. Aims To develop a new classification system for acute UCL injuries of the first MCPj that comprehensively defines all possible combinations and types of injuries and facilitates treatment decision-making for each specific type in daily clinical practice. Methods The retrospective study included patients with acute UCL lesions who underwent conservative or operative treatment at our clinic between 2018 and 2023. This classification addresses ligamentous or osseous lesions, fracture fragment size and dislocation, ligament configuration (e.g. Stener lesions), and joint stability. Correlated radiography, ultrasonography, MRI, and intraoperative findings were used to define all combinations and variants of skier`s thumb injuries. Pre-therapeutic joint stability was characterized according to the criteria established in the literature. Dislocation criteria for bone lesions were defined in a preliminary observer trial. The classification was applied by three hand surgeons with different training levels. Treatment applied and a 3-months follow-up was documented. Results In 213 patients with acute UCL tears, the new classification was consistently applied, and the treatment derived was largely aligned with the recommendations in the literature for both absolute and relative indications for splint or operative therapy. The vast majority of the patients were stable after 3 months. Conclusion This new classification is the first to combine the clinical and radiological aspects of skier`s thumb injuries into one classification and encompasses all variants. It is comprehensive, can be easily applied retrospectively, and provides clear treatment recommendations in most cases.