Purpose To investigate the success rate of the Tübingen hip flexion splint (THFS) for the treatment of developmental dysplasia of the hip (DDH), of different severity as per the Graf classification, among infants younger than six months of age. The type and incidence rate of complications associated with THFS treatment were also evaluated. Methods The following databases were searched using keywords and limited for age less than six months: PubMed, Embase, Web of Science, Cochrane Library, and SinoMed, between inception and July 2020. Articles were screened and extracted by two researchers, and the quality of the included literature was evaluated (methodological index for non-randomized studies criteria). R studio 1.3 was used for statistical analysis. The review process was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Results After screening, eight articles were included in the analysis, contributing 1211 hips (875 patients). The overall success rate of THFS treatment is 91% (95% confidence interval (CI) 0.82 to 0.95). The success rate by Graf type is as follows: type-II, 98% (95% CI 0.94 to 1.00); type-III, 96% (95% CI 0.88 to 1.00); and type-IV, 32% (95% CI 0.18 to 0.47). Complications (24/1211, 2%) included transient femoral nerve palsy (n = 1); avascular necrosis of the femoral head (n = 9) and residual acetabular dysplasia (n = 14). Conclusion THFS treatment is successful for Graf type-II and –III, but low for type-IV, with a low rate of complication. THFS may be an effective treatment option for DDH among infants less than six months of age. However, those with Graf type-IV require close monitoring. Level of Evidence: III