With no serious opposition left, the former Massachusetts governor easily won both contests. He won all 42 delegates at stake in Kentucky and at least 21 of the 33 delegates at stake in Arkansas.
Twelve delegates were still undecided in Arkansas.
Romney has 1,055 delegates, leaving him just 89 shy of the 1,144 delegates needed to win the GOP nomination for president. He should reach the threshold next week, when voters go to the polls in Texas.
Assured of the party nod, Romney has been in general election mode for weeks. He's been spending much of his time fundraising and focusing on Democratic President Barack Obama.
As voters in the two Southern states weighed in, Romney spent Tuesday evening at a fundraising event in New York where his campaign said he raised $5 million. Romney's campaign has raised roughly $15 million during a three-day fundraising swing in the New York area.
He is scheduled to make a campaign appearance Wednesday in Washington.
Romney had struggled in some previous Southern contests, when former Pennsylvania Sen. Rick Santorum and former House Speaker Newt Gingrich were in the race. With them on the sidelines, Romney displayed solid support in two states he should win in November.
Even though they have left the race, Santorum, Gingrich and Texas Rep. Ron Paul were on the ballot in Kentucky and Arkansas.
Texas, which offers 152 delegates, votes May 29.