To estimate international and national temporal trends in the cardiorespiratory fitness (CRF) of adults, and to examine relationships between trends in CRF and trends in health-related, socioeconomic, and environmental indicators. Data were obtained from a systematic search of studies that explicitly reported temporal trends in the CRF of apparently healthy adults aged 18-59 years. Sample-weighted temporal trends were estimated using best-fitting regression models relating the year of testing to mean CRF. Post-stratified population-weighted mean changes in percent and standardized CRF were estimated. Pearson's correlations were used to describe associations between linear trends in CRF and linear trends in health-related, socioeconomic, and environmental indicators. 2,525,827 adults representing eight high- and upper-middle-income countries between 1967 and 2016 collectively showed a moderate decline of 7.7% (95% CI - 8.4 to - 7.0) or 1.6% per decade (95% CI - 1.7 to - 1.5). Internationally, CRF improved in the 1960s and 1970s, and progressively declined at an increasing rate thereafter. Declines were larger for men than for women, and for young adults (< 40 years) than for middle-aged adults (≥ 40 years). All countries experienced declines in CRF with a very strong negative correlation between CRF trends and obesity trends. There has been a meaningful decline in the CRF of adults since 1980, which has progressively increased in magnitude over time, suggestive of a corresponding decline in population health. Continuous national and international surveillance systems are needed in order to monitor health and fitness trends, especially among low- and middle-income countries for which data do not currently exist. PROSPERO registration number: CRD42013003678.