Howard Marks believes investors should be "defensive" due to high valuations and avoid betting for a runaway market.

"Most valuation parameters are either the richest ever … or among the highest in history," Marks wrote in a note to clients Tuesday. "In the past, levels like these were followed by downturns. Thus a decision to invest today has to rely on the belief that 'it's different this time.'"

Marks, whose firm, Oaktree Capital, had $100 billion of assets under management as of September, noted the S&P 500 has roughly quadrupled since its low in 2009 and nearly every valuation metric is high on a historical basis.

On whether investors should be bullish or bearish in the current environment the investor added, "my answer today, as readers know, is that I would favor the defensive or cautious part of the spectrum."

"I'm convinced the easy money has been made," he said.

Marks is known for his prescient investment memos, which warned about the financial crisis and the dot-com bubble implosion.

The Oaktree co-chairman warned his clients from chasing the market in anticipation of a late-stage surge, saying trying to time investor sentiment is "unpredictable."

"It appears many investment decisions are being made today on … belief that the overpriced market may have further to go, and FOMO [fear of missing out]," Marks wrote. "Oaktree will continue to invest on the basis of value and its relationship to price, and to refrain from trying to time markets based on predictions regarding economies, markets or psychology. The 'melt-up' school says securities that already are highly priced may become more so. We'd never bet on whether they will or won't."

On the flip side Bridgewater Associates founder Ray Dalio predicted on Tuesday "a market blowoff" rally, fueled by cash from banks, corporations and investors.

"There is a lot of cash on the sidelines. ... We're going to be inundated with cash," Dalio said. "If you're holding cash, you're going to feel pretty stupid."