The victory -- making Walker the first U.S. governor to survive a recall election -- "will echo beyond the borders of Wisconsin," Romney said in a statement.
"Gov. Walker has shown that citizens and taxpayers can fight back -- and prevail -- against the runaway government costs imposed by labor bosses," the statement said.
Romney's statement echoed a victory speech Walker made after defeating Democratic Milwaukee Mayor Tom Barrett 53 percent to 46 percent.
"Tonight we tell Wisconsin, we tell our country, and we tell people all across the globe, that voters really do want leaders that stand up and make the tough decisions," Walker, 44, declared to a raucous, overflow crowd at an exposition center near Milwaukee around 11:30 p.m. CDT Tuesday (12:30 a.m. EDT Wednesday).
He said he would meet with his Cabinet Wednesday to continue working on behalf of the state's economy.
"We'll renew our commitment to help small businesses grow jobs in the state," he said.
He said he hoped soon to bring Democratic and Republican lawmakers together to meet over burgers and beer. He cut off the crowd when they booed a mention of Barrett, saying that starting Wednesday "we are no longer opponents -- we are one as Wisconsinites."
Barrett, 58, called Walker to concede around 10 p.m., about an hour after TV networks called the race for Walker, the Milwaukee Journal Sentinel reported.
Barrett later told subdued backers at a downtown Milwaukee hotel the loss was "not an end."
"There are more chapters to come," he said. "In those chapters, it is my sincere hope that all of us here will remain engaged.
"Please, please, please remain engaged, remain involved, because we will continue to fight for justice and fairness in this city and state," he said.
"Never ever stop doing what you think is right. That's what makes this such a great country."
The vote capped a contentious 15-month battle that polarized Wisconsin, long a centrist state.
Union members and Democrats protested angrily against Walker's "repairing" of the state budget in March 2011, a process that involved stripping public employees of collective-bargaining rights and benefits.
The race drew more than $63 million in spending by the campaigns and their allied political action committees -- nearly half the money came from outside Wisconsin. Mother Jones reported two-thirds of Walker's funding came from out of state, compared to one-fourth for Barrett.
There have been two other gubernatorial recall elections in U.S. history. Republican North Dakota Gov. Lynn Frazier was recalled in 1921 during a dispute about state-owned industries and Democratic California Gov. Gray Davis was recalled in 2003 over state budget and tax issues.