Applying Rusbult's investment model of dyadic relationships, we examined the effect of caregiver–care recipient relationship closeness (RC) on cognitive and functional decline in Alzheimer's disease. After diagnosis, 167 participants completed up to six visits, observed over an average of 20 months. Participants were 64% women, had a mean age of 86 years, and mean dementia duration of 4 years. Caregiver-rated closeness was measured using a six-item scale. In mixed models adjusted for dementia severity, dyads with higher levels of closeness (p < .05) and with spouse caregivers (p = .01) had slower cognitive decline. Effect of higher RC on functional decline was greater with spouse caregivers (p = .007). These findings of attenuated Alzheimer's dementia (AD) decline with closer relationships, particularly with spouse caregivers, are consistent with investment theory. Future interventions designed to enhance the caregiving dyadic relationship may help slow decline in AD.