Most studies investigating physical activity and cancer risk used a single-time measure of physical activity. The present analysis investigates whether changes in physical activity during midlife influence cancer risk. A prospective cohort of adults ≥40 years from the UK Biobank who provided self-reported physical activity data via the International Physical Activity Questionnaire at both baseline (2007-2010) and follow-up (2012-2013) was analyzed. Changes in physical activity were classified as decreased, maintained, or increased between these two time points. Incident cancers were ascertained up to May 13, 2022. Multivariable Cox regressions were used to examine the associations between changes in physical activity and the risk of cancer overall, by sex, and by obesity- versus nonobesity-related cancers and for breast and prostate cancers. A total of 16,792 participants [mean (SD) age, 56.8 (7.4); 8,421 (50.2%) females] provided repeated data on physical activity. During a median follow-up of 8.3 years, 1,397 incident cancer cases occurred. No statistically significant associations between changes in physical activity and overall cancer risk were found. Increasing physical activity from low to higher levels was associated with a lowered risk of obesity-related cancer [HR = 0.72; 95% confidence interval (CI), 0.54-0.95], particularly for the risk of breast cancer (HR = 0.61; 95% CI, 0.36-1.04). Females who decreased their physical activity from high to lower levels had an elevated cancer risk (HR = 1.47; 95% CI, 1.02-2.11). Increasing physical activity over time was associated with a lower risk of developing obesity-related cancers, particularly breast cancer. Our findings suggest that behavioral changes to increase physical activity in midlife may help reduce cancer risk.