Background: To evaluate how COVID-19 pandemic impacted low back pain (LBP) outcomes in southern Brazil. Methods: Data from the Prospective Study about Mental and Physical Health (PAMPA) Cohort were analyzed. Adults were recruited between June and July 2020 in the Rio Grande do Sul state using online-based strategies. Participants responded a self-reported, online questionnaire on LBP with two timepoints: before (retrospectively) and during COVID-19 pandemic. We assessed the presence of LBP, LBP-related activity limitation (no/yes), and LBP intensity (0 to 10 [strongest pain]). Results: From a total sample of 2,321 respondents (mean age: 37.6 ± 13.5; 75.4% women), the prevalence of LBP did not change significantly from before (74.7% [95%CI 72.3; 76.9]) to during the COVID-19 pandemic (74.2% [95%CI 71.9; 76.3]). However, an increased pain levels (β: 0.40; 95%CI 0.22; 0.58) and a higher likelihood for activity limitation due to LBP was observed during the pandemic (PR 1.14; 95%CI 1.01; 1.29). Longitudinal analyzes showed that age, gender, BMI, chronic diseases, physical activity and anxiety and depression symptoms, were associated with LBP during the COVID-19 pandemic. Conclusion: Although the prevalence of LBP did not change during the COVID-19 pandemic, LBP-induced impairment in daily activities and LBP intensity was higher during than before the pandemic. Sociodemographic and lifestyle characteristics were associated with increased LBP and related outcomes.