Harris, who is also founder of Personal Capital Corporation, came out swinging against bitcoin in an April op-ed in which he called the cryptocurrency a "colossal pump-and-dump scheme, the likes of which the world has never seen."
Harris said Tuesday that bitcoin's value will continue to dwindle until it gets "eventually a whole lot closer to zero."
"We've got digital currencies. And we've got digital currencies that are more stable, more widely accepted and have intrinsic value. We've already got it — it's called the dollar, the yen, you name it," Harris said.
The former financial technology CEO's primary concerns about bitcoin include its slow transaction time, scaling challenges and volatility, which "alone makes it useless as a payment mechanism and ridiculous as a store of value," Harris said.
"There has to be something underpinning it," he said. "Bitcoin makes no revenue, no profitability."
Bitcoin climbed more than 1,300 percent in 2017 to nearly $20,000, then lost almost half its value in the first three months of 2018. Bitcoin dipped below $6,000 on Tuesday for the first time since June, before paring some losses to trade near $6,114 as of 6 p.m. EST, according to CoinDesk.
Some cryptocurrency enthusiasts and investors have attributed the sell-off to kickback from Securities and Exchange Commission activity. The SEC moved to delay a decision on the VanEck SolidX Bitcoin Trust ETF on Aug. 7, after it rejected a separate ETF proposal from Cameron and Tyler Winklevoss on July 26.