Although Romney noted that he is in favor of the first charge, he went on to add that he would vote against the second article of impeachment - obstruction of Congress - because not enough evidence was secured to make the case.
"The grave question the Constitution tasks senators to answer is whether the president committed an act so extreme and egregious that it rises to the level of a high crime and misdemeanor," Romney said. "Yes, he did."
“With my vote I will tell my children and their children that I did my duty ... What the president did was wrong. Grievously wrong," he added.
Romney's announcement marks the first time in US history that a senator has voted to convict a president of the same political party in an impeachment trial.
In remarks to the New York Times ahead of his Senate floor appearance, Romney stated that the repercussions for his stance would be "unimaginable."
“I recognize there is going to be enormous consequences for having reached this conclusion,” he said.
Moments after Romney declared his move, the White House announced that it would no longer be inviting reporters to the Oval Office for an already scheduled pool spray. CNBC's White House correspondent Eamon Javers tweeted that the call was sent out when reporters "were already on the South Lawn and ready to go in."
The final vote on both articles of impeachment is due to come down later Wednesday, days after the body voted against calling additional witnesses in the impeachment trial. That vote came down 51-49.