Purpose: Studies have shown that elongation of the injured Achilles tendon after acute Achilles tendon rupture (ATR) is negatively associated with clinical outcomes. The difference between operative and non-operative treatment on the length of the Achilles tendon is only sparsely investigated. The aim of the study was to investigate if the operative and non-operative treatment of ATR had different effects on tendon elongation. Methods: The study was performed as a registry study in the Danish Achilles tendon database (DADB). The primary outcome of the study was an indirect measure of Achilles tendon length: the Achilles tendon resting angle (ATRA) at 1-year follow-up. The variable of interest was treatment (operative or non-operative). Results: From August 2015 to January 2019, 438 patients (154 operatively treated and 284 non-operatively treated) were registered with full baseline data and had their ATRA correctly registered at 1-year follow-up in DADB. The analysis did not show a clinically relevant nor statistically significant difference in ATRA between operative and non-operatively treated patients at 1-year follow-up (mean difference − 1.2°; 95% CI − 2.5; 0.1; n.s) after adjustment for potential confounders. Conclusion: There were neither clinically relevant nor statistically significant differences in terms of the ATRA at 1-year follow-up between the operative and non-operatively treated patients. This finding suggests that operative treatment does not lead to a clinically relevant reduction in tendon elongation compared to non-operative treatment and it should therefore not be used as an argument in the choice of treatment. Level of evidence: Level III.