Abstract The appendicular extracellular-to-intracellular water ratio (A-E/I) is a potential marker of skeletal muscle quality, reflecting the balance of water distribution between the extracellular and intracellular compartments of the appendicular limb regions. A-E/I has been increasingly used in recent studies; however, its association with adverse outcomes remains unclear. This study investigated the potential association between A-E/I and all-cause mortality. A prospective cohort study of 8 015 middle-aged and older adults (comprised of 4 755 women, aged 45–74 years) residing in a Japanese community was conducted. The baseline assessment was performed between 2010 and 2012, and the follow-up period lasted until July 2022. A-E/I and skeletal muscle mass were measured using segmental bioelectrical impedance spectroscopy. Handgrip strength (HGS) was measured using a Smedley-type dynamometer. Lifestyle, medical history, and physical activity were assessed by questionnaire and accelerometer. Hazard ratios (HRs) and 95% confidence intervals (95% CIs) for each quartile (Q) of A-E/I were estimated using the multivariable Cox regression model. During a 10.5-year median follow-up, the mortality rates were 8.9 and 3.6 per 1 000 person-years for men (292 deaths) and women (174 deaths), respectively. A-E/I quartiles were positively associated with all-cause mortality in both sexes (men: Q1, HR: 1.0 [95% CI: reference], Q4, HR: 1.8 [1.1–2.9], ptrend < .05; women, Q4, HR: 2.2 [1.3–3.8], ptrend < .01). This association remained significant after further adjustment for skeletal muscle mass and HGS (ptrend < .05). Our findings suggest that A-E/I serves as an early predictive marker for mortality in middle-aged and older Japanese adults.