Schumer directly attacked Mnuchin's claims the Republican tax reform plan will reduce the country's budget deficit.
"I don't know if the man's deliberately lying, but it seems so," Schumer said in a "Politico Money" podcast interview. "His statements are outlandish, and he seems just to want to, I don't know if I'm allowed to use this word, I think I am, suck up to Trump, flatter Trump. … This man cannot be believed. And if he used these kinds of arguments at Goldman Sachs, they would have fired him."
Schumer also rejected Mnuchin's prediction of a "significant" market drop if tax reform doesn't pass.
"It's absurd. No one believes Steven Mnuchin," he said. "He has lost his credibility because of statements like that. Wall Street was doing great, it was going up at a dramatic rate before the president even took office. And it's doing fine now, because the economy's doing well, because corporate profits are way up."
The S&P 500 has rallied 20 percent since the Nov. 8 election through Tuesday.
A Treasury spokesperson sent the following statement in response to a request for comment:
"We are surprised to hear the Minority Leader express these comments about the Secretary. The Secretary is committed to delivering a major middle class tax cut, more competitive business taxes that create jobs and wage growth and a fairer tax system. This would be a good time for Senator Schumer to stop the obstruction and help hard working Americans get meaningful tax relief."