Wealthy investors are holding more cash just in case of a full-blown trade war, Ermotti told CNBC's Michelle Caruso-Cabrera in an interview that aired on "Squawk Box." He said he's talking about "something that goes into a territory where the escalation of measures that are taken starts to impact business sentiment."
The tariffs that President Donald Trump announced on Friday against Chinese imports are set to start July 6. China is promising retaliatory measures. The White House also recently hit U.S. allies Canada, Mexico and the European Union with steel and aluminum import tariffs, a move that also was met with retaliatory measures.
"I'm really worried that ... these things are going to get out of control. Somebody is going to announce something that then triggers a more serious issue," Ermotti said, stressing trade risk "could come from any side, Europe, U.S., China; you name it."
Extreme protectionism could become a real problem if there are any other unexpected events such as the Federal Reserve accelerating the path of interest rate hikes, he said.
"Asset classes are priced quite high across the board. Expectations for business profitability are quite high. The financial markets are not ready for any major discontinuity ... in commercial ties between countries," Ermotti said.
The chief executive at UBS, one of the world's biggest managers of billionaires' money, described stocks as not yet priced to perfection but getting close. "You can still see value in equities" for the time being.
Ermotti also told CNBC that UBS is putting more emphasis on catering to high net-worth investors in the U.S. and even more so in Asia.
He said the firm is following the money.
In the Americas, UBS has 7,000 advisors who contribute 33 percent of their profits before taxes. In Asia, the company has fewer advisors, roughly 1,100, who contribute 27 percent of the profits before taxes.
"In Asia, you have to think that we basically bank with one out of two billionaires in the region," Ermotti said. "Every four days in China, there is a new billionaire. Last year, the numbers of billionaires in China really [overtook] the U.S. And in [a] few years' time, they're going to have more wealth than the U.S."
Some of that wealth in Asia may find its way to U.S. and Europe, he said.
"Before a couple of years ago, Asia investors in general were very interested in investing in Asia. And rightly so, they had so much opportunity, he said. "Now you're seeing more and more investors willing to diversify their portfolios across globally in the U.S. or Europe."