Investor sentiment today is looking like it did in 2000, and that could be a sign markets are in a bubble, Yale professor and Nobel economics laureate Robert Shiller told CNBC on Tuesday.

Just before the dot-com bubble burst, investors had very little confidence in stock valuations, but they were confident in the market in the short term, he said.

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"That's the sign of the bubble. They're worried but they're thinking they'll get out," he told CNBC's "Squawk Box." "This can suddenly turn, and we're looking somewhat like that now."

Shiller raised concerns about the value of U.S. stocks earlier this week when he told the Financial Times his valuation confidence index showed investors believed equities have not been so overvalued since 2000, when the dot-com bubble burst.

The index, based on investor surveys, and also known as the CAPE ratio, compares current stock prices to earnings over the course of 10 years to account for business cycles. CAPE stands for cyclically adjusted price-earnings ratio.

The CAPE ratio is currently at 25, Shiller said, above the historical average of 17. In 2000, it spiked as high as 44 before dropping back down to 13, Shiller told CNBC earlier this month.

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Shiller said he did not want to be too negative, but said he is more worried about a decline than most people.

"I think the market might do what it did in 2000, which is the CAPE ratio went up. It's 25 now. It could go up to 45," he said.