"The premise of that article is false, that in any way the president had a conversation that was inappropriate or that resulted in any kind of lapse in national security," H.R. McMaster told a White House briefing.
According to the Post, Trump revealed highly classified details of an ISIS terror threat during a meeting last week with Russian Foreign Minister Sergei Lavrov and Moscow's ambassador in Washington, Sergey Kislyak.
The information reportedly came from a US ally who had not authorized Washington to share it with Moscow.
McMaster would neither confirm nor deny that Trump had divulged top secret information.
"What we don't do is discuss what is what isn't classified," he said.
"What I will tell you is in the context of that discussion, what the president discussed with the foreign minister was appropriate to that conversation and is consistent with the routine sharing of information between the president and any leaders with whom he's engaged."
But when asked about concerns the president could have jeopardized US intelligence relationships, McMaster responded: "I'm not concerned at all."
"It is wholly appropriate for the president to share whatever information he thinks is necessary to advance the security of the American people. That's what he did."
Trump, he said, had been underscoring the need for cooperation in tackling ISIS, even as he pushed Lavrov on the need for Russia to change its behavior in Ukraine and Syria.
"This was the context of the conversation in which it was wholly appropriate to share what the threat was as a basis for common action and coordination," McMaster said.
US lawmakers on both sides of the political aisle have voiced astonishment at the allegations, and demanded a full explanation from the White House.