Predictive clinical and radiological factors can potentially identify adolescent idiopathic scoliosis (AIS) most likely to benefit from overcorrection nighttime bracing. These factors can provide helpful information in clinical decision making. However, the relationship between these factors and outcomes of overcorrection nighttime bracing is unclear. This systematic review determined the predictive factors for identifying outcomes of overcorrection nighttime bracing in AIS. A systematic search was conducted on PubMed, MEDLINE, Scopus, and Embase from January 1986 to January 2021. Studies on AIS patients, aged 10–18 years, with a Risser sign of 0–2 and an initial Cobb angle of 20°–45°, who were treated with overcorrection nighttime bracing and for whom at least one predictive factor of treatment outcome (failure and/or success) was assessed were included. Two blinded reviewers independently evaluated the studies using a quality assessment tool. To determine predictive factors, the level of evidence was rated through best-evidence synthesis. A total of nine studies met the inclusion criteria. A Providence brace was used in six of the included studies, while a Charleston bending brace was used in three. Findings from two high-quality studies provided strong evidence of the association between curve flexibility and brace treatment success. In terms of the Risser sign, this evidence was obtained from three high-quality studies. Moderate evidence indicated a positive association between premenarchal status and nighttime bracing failure. Inconclusive evidence indicated that poor brace compliance is associated with treatment failure. Conflicting evidence of treatment failure was indicated for initial curve magnitude, curve type, in-brace correction, age, Risser sign, curve apex, and sex. These findings show that greater curve flexibility and a higher Risser sign are associated with overcorrection nighttime bracing success.