"This is not acceptable", wrote Donald Trump in Twitter.
China has historically manipulated its currency for its own benefit.
"China has devalued their currency in the past - as a matter of fact, through 2016 they devalued it significantly and starting in 2017, since the president was elected, they've used a lot of their reserves to actually support their currency", Mnuchin said early Tuesday in a CNBC interview.
The U.S. Treasury, in a semiannual report on Friday, again refrained from naming any major trading partners as currency manipulators.
A Treasury spokeswoman referred questions to the White House, which offered little clarification on the apparent contradiction.
President Donald Trump pushed back against the Russian and Chinese governments Monday morning, calling them out for reportedly devaluing their currencies against the US dollar.
Trump's criticism of Chinese trade and economic policy-and the USA politicians who he said enabled it- was a centerpiece of his campaign. Trump restated that America is not going to name China as the currency manipulator.
Asked whether Trump's tweet on Monday about China and Russian Federation being currency manipulators had to do with wanting a weaker dollar, Mnuchin said: "That's not the case".
Democrats and Republicans in the past have accused Beijing of improperly devaluing its currency in a way that makes Chinese exports more attractive to foreign buyers, but China has recently moved away from this in the face of enormous worldwide pressure.
The US President's statement is a reference to what he regards as an unfair commercial advantage - if the currency of a country is artificially lowered, its exports are more competitive. According to the report, "China has an extremely large and persistent bilateral trade surplus with the United States, by far the largest among any of the United States' major trading partners". The department is adding India to that list.