The Tegner Activity Scale (TAS) has been used in several Danish studies. However, the TAS has not been cross-culturally adapted into Danish. We aimed to translate the TAS into Danish and to evaluate its measurement properties in patients with knee disorders. The TAS was translated into Danish following international guidelines. The measurement properties were evaluated in 86 patients (median: 25 years). For reliability, 56 patients completed the scale twice within 9 days. responsiveness and interpretability were evaluated in 64 patients completing the TAS 6 months later. For reliability, the intraclass correlation coefficient was 0.7 (CI: 0.6-0.8),the standard error of measurement was 0.7 points (CI: 0.6-0.9), the smallest detectable change was 2.1 points (CI: 1.7-2.5), and the limits of agreement were −1.7-2.4 points. For responsiveness, the correlation between the TAS change score and the Global Rating of Change Scale was 0.3 (CI: 0.1-0.5). In patients reporting improved knee condition, 55% reported a TAS change score of minimum 1 point. For interpretability, the minimal clinically important change was 0.7 points (SD: 1.4), and no floor or ceiling effects were found. In conclusion, the Danish version of the TAS has acceptable test-retest reliability. However, the TAS seems less adequate to evaluate changes over time.