Snyder's signature at the ceremony at Oakwood Hospital in Dearborn makes Michigan the largest state controlled by Republicans to support a key component of the new federal health care law.
It will expand the government health insurance program to almost a half-million Michigan residents within a few years, nearly halving the state's uninsured. An estimated 320,000 are expected to be eligible in the spring if the federal government approves the plan.
"This is something that's really important. This is about the health of our fellow Michiganders," Snyder said.
Medicaid expansion is part of a strategy to ensure that nearly all Americans have health insurance under the Affordable Care Act. It was designed to cover the neediest uninsured people yet became optional for states because of a Supreme Court decision.