Climate change is amplifying heat exposure worldwide; however, its consequences for global physical inactivity, and the resulting effects on mortality and economic burden, remain unquantified. We analysed a longitudinal dataset spanning 156 countries from 2000 to 2022 using a binned fixed-effects panel regression model. The model examined the relationship between the primary outcome-the age-standardised prevalence of physical inactivity in adults (aged ≥18 years)-and annual exposure to different temperature ranges. Estimated exposure coefficients and climate projections under different shared socioeconomic pathways (SSPs) were used to forecast future physical inactivity. Using relative-risk estimates for all-cause mortality, we converted projected physical inactivity into excess deaths and valued lost productivity using a friction-cost approach calibrated to each country's gross domestic product and labour participation rates. Each additional month with a mean temperature >27·8°C increased physical inactivity by 1·44 (95% CI 0·49-2·39) percentage points globally and 1·85 (0·62-3·08) percentage points in low-income and middle-income countries. By 2050, the prevalence of physical inactivity is projected to rise by 0·98 (0·47-1·49) percentage points under SSP1-2.6, 1·22 (0·58-1·85) percentage points under SSP2-4.5, and 1·75 (0·84-2·66) percentage points under SSP5-8.5, with hotspots exceeding 4 percentage points in Central America, the Caribbean, eastern sub-Saharan Africa, and equatorial southeast Asia. By 2050, these increases translate into an additional 0·47-0·70 million deaths and Intl$2·40-3·68 billion in annual productivity losses. Rising temperatures are projected to increase the prevalence of physical inactivity, translating into additional premature deaths and productivity losses, especially in tropical regions. Prioritising heat-adaptive urban design, subsidised climate-controlled exercise facilities, and targeted heat-risk communication is essential to mitigate these emerging health and economic burdens, in addition to ambitious emissions reductions. Wellcome Trust 304972/Z/23/Z (Lancet Countdown Latin America).