MIAMI — Gov. Rick Scott of Florida reversed himself on Wednesday and announced that he would expand his state’s Medicaid program to cover the poor, becoming the latest — and, perhaps, most prominent — Republican critic of President Obama’s health care law to decide to put it into effect.

It was an about-face for Mr. Scott, a former businessman who entered politics as a critic of Mr. Obama’s health care proposals. Florida was one of the states that sued to try to block the law. After the Supreme Court ruled last year that though the law was constitutional, states could choose not to expand their Medicaid programs to cover the poor, Mr. Scott said that Florida would not expand its programs.

Mr. Scott said Wednesday that he now supported a three-year expansion of Medicaid, through the period that the federal government has agreed to pay the full cost of the expansion, and before some of the costs are shifted to the states.

“While the federal government is committed to paying 100 percent of the cost, I cannot in good conscience deny Floridians that needed access to health care,” Mr. Scott said at a news conference. “We will support a three-year expansion of the Medicaid program under the new health care law as long as the federal government meets their commitment to pay 100 percent of the cost during that time.”