Background: Quarantine periods change routines and behaviors with potential impact on different health outcomes. Objective: To determine the association between changes in physical activity and sedentary behaviors with changes in back pain during the COVID-19 pandemic quarantine among Brazilian adults. Methods: This was a nationwide survey through online questionnaires using data from 43,062 adults (≥ 18 years of age). Information on back pain was assessed using questions about episodes of back pain and worsening symptoms during the quarantine. The pattern of movement behaviors adopted before and during the pandemic were considered for physical inactivity (< 150 min/week of activity), high TV-viewing (≥ 4 h/d), and high computer/tablet use (≥ 4 h/ d). Covariates included sex, age group, academic achievement, skin color, working status during the quarantine, and adherence to the quarantine. Logistic regression models were used for statistical analyses (weighted for national representativity). Results: Becoming inactive (OR=1.76, 95% CI: 1.32, 2.37), with high TV-viewing (OR=1.35, 95% CI: 1.14, 1.61) and high computer/tablet use (OR=1.39, 95% CI: 1.11, 1.73) during the pandemic were associated with a higher incidence of back pain. The incidence of physical inactivity was also associated with increased back pain (OR=2.71, 95%CI: 1.64, 4.48). Conclusions: We conclude that increased physical inactivity and sedentary behaviors due to the COVID-19 pandemic quarantine are associated with the incidence and worsening symptoms of back pain among Brazilian adults.