This study explored the association between the Frailty Index (FI) and low back pain (LBP) in middle-aged and older Chinese adults. We hypothesised that a higher FI correlates with increased LBP prevalence.Cross-sectional analysis.The study used data from the China Health and Retirement Longitudinal Study (CHARLS) across various regions of China.The analysis included 6375 participants aged 45 and above with complete LBP and FI data from the CHARLS for 2011, 2013 and 2015. We excluded individuals under 45, those with incomplete LBP data, participants with fewer than 30 health deficit items and those missing covariate data.We constructed an FI consisting of 35 health deficits. Logistic multivariable regression examined the relationship between FI and LBP, using threshold analysis to identify inflection points. Sensitivity analyses were performed to ensure the robustness of the findings.Of the participants, 27.2% reported LBP. A U-shaped association was observed between FI and LBP, with the highest quartile (Q4, FI ≥0.23) showing more than a twofold increased risk of LBP (OR=2.90, 95% CI: 2.45-3.42, p<0.001). Stratified analysis showed a significant association in participants under 60, particularly in the lowest FI quartile (OR=1.43, 95% CI: 1.14 to 1.79). Sensitivity analysis upheld the robustness of the primary results.The findings suggest a complex relationship between frailty and LBP, highlighting the need for early screening and tailored interventions to manage LBP in this demographic. Further research is necessary to understand the mechanisms of this association and to validate the findings through longitudinal studies.© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.