"Are we in a market bubble or not? I don't know. I'm silent on it. Where multiples are, etc., you could be a little worried," Ballmer said on CNBC's "Squawk Box."
"I killed myself in the late '90s when I said the stock market was overdone and we got a hiccup down the next day," he said.
When asked whether President Donald Trump's pro-business policies have contributed to the market's gains, he said: "I have no idea."