Introduction Increased gut permeability (“leaky gut”) has been proposed as a potential contributor to age-related inflammation and gut dysbiosis. However, information on the relationship between a leaky gut and inflammation and physical frailty during aging are limited. Objective To investigate the hypothesis that an aging-associated leaky gut is linked to the age-related inflammation and frailty. Methods Two cohorts of healthy adults were studied: young (18–30 years old, n = 19) and older (≥70 years old, n = 18). Serum concentrations of the tumor necrosis factor (TNF)-α and interleukin (IL)-6, zonulin (a marker for leaky gut), and high-mobility group box protein (HMGB1, a nuclear protein triggering inflammation) were measured. Correlations of serum levels of zonulin and HMGB1 with strength of plantar flexor muscles and number of steps taken per day were analyzed. Results Serum concentration of zonulin and HMGB1 were 22% (P =.005) and 16% (P =.010) higher in the older versus young adults. Serum zonulin was positively associated with concentrations of TNF-α (r = 0.357, P =.032) and IL-6 (r = 0.345, P =.043). Importantly, both zonulin and HMGB1 were negatively correlated with skeletal muscle strength (zonulin: r = −0.332, P =.048; HMGB1: r = −0.383, P =.023), and habitual physical activity (zonulin: r = −0.410, P =.016; HMGB1: r = −0.483, P =.004). Conclusions Serum zonulin was associated with both systemic inflammation and 2 key indices of physical frailty. These data suggest that a leaky gut may play a critical role in the development of age-related inflammation and frailty.