OBJECTIVE Several reviews have summarised studies on secondary school students' moderate-to-vigorous physical activity (MVPA) in physical education (PE), but no systematic review with semiquantitative assessment has been conducted to specifically identify the correlates of their MVPA. This review aims to systematically summarise the existing literature, which investigated correlates of MVPA of secondary school students during their PE lessons. METHODS A systematic search using ERIC, SPORTDiscus, PubMed, PsycINFO, Academic Search Premier, and Web of Science was conducted to identify the correlates of the MVPA of secondary school students in PE. Studies were eligible if they were English published articles and examined the association with MVPA during secondary school PE lessons and cross-sectional and prospective longitudinal quantitative studies. Two reviewers independently examined the articles, assessed their methodological quality, and performed data extraction. The correlates of MVPA were synthesised and further assessed semiquantitatively. RESULTS Fifty-five studies were identified to correlate with secondary school students' MVPA in PE lessons. Further analysis only included 43 studies (78.2%) that were of medium and high quality by methodological quality assessment. Out of 54 variables identified from these medium and high-quality studies, 11 were consistently associated with the MVPA. Sex (boys), ethnicity (White), class gender (boys-only), PE activities (team games), lesson location (outdoors), expectancy beliefs, subjective task values, and enjoyment were consistently and positively associated with MVPA. Other variables, namely, class gender (girls-only), PE activities (movement activities), and lesson context (knowledge), were consistently and negatively related to MVPA. CONCLUSIONS Interventions focusing on the consistent variables are needed to build active lesson time in PE. This review also provides insights for future research.