"I am honored that Americans across the country have given their support to my candidacy and I am humbled to have won enough delegates to become the Republican Party's 2012 presidential nominee," Romney said in a statement after claiming 70 percent of the Texas vote to get at least 78 delegates he needed to go over the threshold of 1,144 delegates needed for the nomination.
U.S. Rep. Ron Paul of Texas got 11 percent of the vote.
"Our party has come together with the goal of putting the failures of the last 3 1/2 years behind us," said Romney, the first Mormon to win a major party's nomination. "I have no illusions about the difficulties of the task before us. But whatever challenges lie ahead, we will settle for nothing less than getting America back on the path to full employment and prosperity."
Romney attended a Las Vegas fundraiser Tuesday with real estate mogul and reality TV personality Donald Trump, who in interviews this week once again contended Obama was born in a foreign country, despite evidence the president was born in Hawaii.
Romney didn't condemn Trump's statements, prompting Obama deputy campaign manager Stephanie Cutter to allege Romney lacked "moral leadership" for his "refusal to condemn [Trump's] disgraceful conspiracy theories."
She called Trump a "charlatan."
When Romney was asked about Trump's "birther" views Monday night, he said: "I don't agree with all the people who support me and my guess is they don't all agree with everything I believe in. But I need to get 50.1 percent or more, and I'm appreciative to have the help of a lot of good people."
Romney's campaign later stated directly the candidate did not share Trump's skepticism about the authenticity of the president's birth certificate.
In another Texas primary vote, Texas Lt. Gov. David Dewhurst and Tea Party-supported lawyer Ted Cruz were headed for a runoff to choose the GOP nominee to succeed retiring U.S. Sen. Kay Bailey Hutchison.
Dewhurst, whose supporters included Gov. Rick Perry, is to face the former Texas solicitor general in a runoff July 31.
Former Alaska Gov. Sarah Palin -- the 2008 Republican vice presidential nominee -- supported Cruz.