Introduction Side-to-side strength differences are often used in return to sport (RTS) decision making after hamstring injury. In football (soccer), there is little consensus, and a side-to-side difference of <10% is often suggested as a criterion. To date, no study has determined whether differences exist when comparing the strength of the injured limb at RTS with the preinjury level. Purpose The aims of this study were to compare the isokinetic strength at RTS with preinjury strength in the injured limb and to investigate the side-to-side differences at RTS. Methods This study was based on a pooled group of patients participating in two single-site randomized controlled trials. All eligible professional male soccer players who suffered a hamstring injury confirmed by magnetic resonance imaging were included. The isokinetic strength of the injured limb at RTS was compared with preinjury screening tests, and the side-to-side difference was determined at both time points. Results In total, 41 hamstring injuries were included in the analyses. Side-to-side differences in isokinetic strength at RTS were similar to preinjury levels. At RTS, the average strength of the injured limb was >95% compared with preinjury strength. Overall, 63.4% of the players had a >10% negative strength difference (i.e., the injured limb being weaker), and 57.9% of the players had a >10% positive strength difference (i.e., the injured limb being stronger) in at least one of the five isokinetic variables. Conclusion The distribution of isokinetic strength differences when comparing strength at RTS with preinjury measures was similar, urging clinical caution when interpreting these results. Small changes in the isokinetic strength of the injured limb at RTS compared with preinjury strength suggest that isokinetic cutoff values are not useful to guide the restoration of strength as a criterion for RTS after hamstring injury.