Shiller's comment came after he told CNBC PRO in an exclusive interview that investors should stay in the market because it "could go up 50 percent from here."
"We have maybe inspiration from the White House, a businessman president," the Sterling Professor of Economics at Yale University said on "Halftime Report." "If factors go right and there are tax cuts for corporations, it's not that hard to understand that that could happen."
Shiller CAPE PE Ratio
Still, Shiller said the market could surge to that percentage on its own.
In the interview with CNBC PRO, Shiller said of the president: "He is an inspiration for many people. And many other people don't like him, as you may have heard. On the other hand, they may still invest in a rising stock market out of belief in his powers." (Read the full CNBC PRO report and see the interview video here.)
Shiller, who developed the "cyclically adjusted price-to-earnings ratio" (CAPE) market valuation measure, said that he didn't mean to cause "jaws to drop" with his prediction, but added stocks have generally outperformed other investments through history.
Stocks are "highly priced now, which means I don't expect them to outperform so much," he said. "But for a long-term investor and most people are, I think there should be a place for stocks in the portfolio and they could go up a lot from where they are now ... they could also go down."
— CNBC's Tae Kim contributed to this report.