The official confirmed to ABC News that the party will be sending resources to Alabama in the final week of the race after pulling funding in November over allegations of sexual misconduct by Moore.
Moore has faced allegations from at least eight women in recent weeks who have accused him of sexual misconduct or inappropriate behavior toward them when he was in his 30s and, in some cases, the women were in their teens.
He has vehemently denied all of the allegations, and openly attacked the women as liars at campaign events over the past month.
The National Republican Senate Committee will remain on the sidelines of the race.
The special election will be held Dec. 12.
The reversal by the RNC comes just eight days after Chairwoman Ronna McDaniel told conservative radio host John Catsimatidis' show, "The allegations were obviously very concerning, and concerning to the degree that we pulled our resources. ... The Alabama voters are going to have to be the judge and jury on this."
Trump, who had straddled a line between outright support of Moore for weeks, officially said he was backing the candidate on Monday. Trump tweeted his support for Moore on Monday morning and the White House later confirmed the two had spoken on the phone.
Moore released a statement on Monday promoting the president's endorsement.
"I am honored to receive the support and endorsement of President Donald Trump," Moore said in the statement. "President Trump knows that the future of his conservative agenda in Congress hinges on this election. I look forward to fighting alongside the president to strengthen our military, secure our border, protect our gun rights, defend the sanctity of life and confirm conservative judges to courts around this nation."
Trump is not scheduled to campaign for Moore, though he will appear at a rally in Pensacola, Florida on Friday about a half hour from the Alabama state line.