Donald Trump said on Monday that he was actively considering breaking up the biggest banks on Wall Street — a message that briefly sent markets down sharply.

Trump appeared to throw new weight behind a revived version of the Glass-Steagall Act, which kept investment banks from merging with commercial banks, but was dismantled in 1999 after the merger of Citibank and Traveler’s Insurance.

“There’s some people that want to go back to the old system, right? So we’re going to look at that,” Trump said in an interview with Bloomberg News.

The report was greeted with skepticism.

“Jesus, I can’t keep up with this guy,” Marcus Stanley, policy director at Americans for Financial Reform, told The Post.

“The devil is very much in the details of these kinds of things, so in the absence of those details, it’s very difficult to make any kind of judgment,” he added.

The report of the interview sent the Dow Jones industrial average skidding into negative territory, down to 20,925.65, before paring losses.

Hours before Trump made the breakup pronouncement, Treasury Secretary Steve Mnuchin told a group of bankers in California that they should “thank me for your bank stocks doing better.”

Trump has so far been very good for banks, and has vowed to “do a number” on an Obama-era regulation, the Dodd-Frank Act, which forces banks to reserve more capital in case of disasters, and bars them from trading with their customer’s money.

Goldman Sachs’ stock rose more than 35 percent last year, largely on the back of Trump’ s win. The stock is off about 6 percent so far this year.