Objective: The purpose of this study was to evaluate the impact of objective isolation and loneliness on Medicare spending and outcomes. Method: We linked Health and Retirement Study data to Medicare claims to analyze objective isolation (scaled composite of social contacts and network) and loneliness (positive response to three-item loneliness scale) as predictors of subsequent Medicare spending. In multivariable regression adjusting for health and demographics, we determined marginal differences in Medicare expenditures. Secondary outcomes included spending by setting, and mortality. Results:Objective isolation predicts greater spending, US$1,644 (p < .001) per beneficiary annually, whereas loneliness predicts reduced spending, −US$768 (p < .001). Increased spending concentrated in inpatient and nursing home (skilled nursing facilities [SNFs]) care; despite more health care, objectively isolated beneficiaries had 31% (p < .001) greater risk of death. Loneliness did not predict SNF use or mortality, but predicted slightly less inpatient and outpatient care. Discussion: Objectively isolated seniors have higher Medicare spending, driven by increased hospitalization and institutionalization, and face greater mortality. Policies supporting social connectedness could reap significant savings.