Physiological linkage refers to the degree to which two individuals' central/peripheral physiological activities change in coordinated ways. Previous research has focused primarily on linkage in the autonomic nervous system in laboratory settings, particularly examining how linkage is associated with social behavior and relationship quality. In this study, we examined how linkage in couples' daily somatic activity (e.g., synchronized movement measured from wrist sensors)—another important aspect of peripheral physiology—was associated with relationship quality and mental health. We focused on persons with neurodegenerative diseases (PWNDs) and their spousal caregivers, whose linkage might have direct implications for the PWND–caregiver relationship and caregiver’s health. Twenty-two PWNDs and their caregivers wore wristwatch actigraphy devices that provided continuous measurement of activity over 7 days at home. PWND–caregiver activity linkage was quantified by the degree to which activity was “in-phase” or “anti-phase” linked (i.e., coordinated changes in the same or opposite direction) during waking hours, computed by correlating minute-by-minute activity levels averaged using a 10-min rolling window. Caregivers completed well-validated surveys that assessed their mental health (including anxiety and depression) and relationship quality with the PWND. We found that lower in-phase activity linkage, but not anti-phase linkage, was associated with higher caregiver anxiety. These dyad-level effects were robust, remaining significant after adjusting for somatic activity at the individual level. No effects were found for caregiver depression or relationship quality. These findings suggest activity linkage and wearables may be useful for day-by-day monitoring of vulnerable populations such as family caregivers. We offered several possible explanations for our findings.