BACKGROUND: Low back pain is highly prevalent and the main cause of years lived with disability (YLDs). We present the most up-to-date global, regional, and national data on prevalence and YLDs for low back pain from the Global Burden of Diseases, Injuries, and Risk Factors Study 2021. METHODS: Population-based studies from 1980 to 2019 identified in a systematic review, international surveys, US medical claims data, and dataset contributions by collaborators were used to estimate the prevalence and YLDs for low back pain from 1990 to 2020, for 204 countries and territories. Low back pain was defined as pain between the 12th ribs and the gluteal folds that lasted a day or more; input data using alternative definitions were adjusted in a network meta-regression analysis. Nested Bayesian meta-regression models were used to estimate prevalence and YLDs by age, sex, year, and location. Prevalence was projected to 2050 by running a regression on prevalence rates using Socio-demographic Index as a predictor, then multiplying them by projected population estimates. FINDINGS: In 2020, low back pain affected 619 million (95% uncertainty interval 554-694) people globally, with a projection of 843 million (759-933) prevalent cases by 2050. In 2020, the global age-standardised rate of YLDs was 832 per 100 000 (578-1070). Between 1990 and 2020, age-standardised rates of prevalence and YLDs decreased by 10·4% (10·9-10·0) and 10·5% (11·1-10·0), respectively. A total of 38·8% (28·7-47·0) of YLDs were attributed to occupational factors, smoking, and high BMI. INTERPRETATION: Low back pain remains the leading cause of YLDs globally, and in 2020, there were more than half a billion prevalent cases of low back pain worldwide. While age-standardised rates have decreased modestly over the past three decades, it is projected that globally in 2050, more than 800 million people will have low back pain. Challenges persist in obtaining primary country-level data on low back pain, and there is an urgent need for more high-quality, primary, country-level data on both prevalence and severity distributions to improve accuracy and monitor change. FUNDING: Bill and Melinda Gates Foundation.