IMPORTANCEPostacute care is thought to be a major source of wasteful spending. The extent to which accountable care organizations (ACOs) can limit postacute care spending has implications for the importance and design of other payment models that include postacute care. OBJECTIVE To assess changes in postacute care spending and use of postacute care associated with provider participation as ACOs in the Medicare Shared Savings Program (MSSP) and the pathways by which they occurred. DESIGN, SETTING, AND PARTICIPANTS With the use of fee-for-service Medicare claims from a random 20% sample of beneficiaries with 25 544 650 patient-years, 8 395 426 hospital admissions, and 1 595 352 stays in skilled nursing facilities (SNFs) from January 1, 2009, to December 31, 2014, difference-in-difference comparisons of beneficiaries served by ACOs with beneficiaries served by local non-ACO health care professionals (control group) were performed before vs after entry into the MSSP. Differential changes were estimated separately for cohorts of ACOs entering the MSSP in 2012, 2013, and 2014. EXPOSURES Patient attribution to an ACO in the MSSP. MAIN OUTCOMES AND MEASURES Postacute spending, discharge to a facility, length of SNF stays, readmissions, use of highly rated SNFs, and mortality, adjusted for patient characteristics. RESULTS For the 2012 cohort of 114 ACOs, participation in the MSSP was associated with an overall reduction in postacute spending (differential change in 2014 for ACOs vs control