On CNBC in August, he called bitcoin a "punter's dream."
In November, he said it's for "criminals" and "millennials."
Earlier this month, he admitted new bitcoin futures lend some "legitimacy," but said it's a "modern day tulip bulb mania."
The founder and publisher of The Gartman Letter, a four-decade veteran of the financial markets, told CNBC's "Worldwide Exchange" on Friday that investors should "stay the heck anyway from bitcoin."
Gartman, who made his bones on Wall Street as a commodities trader, said: "It makes no sense whatsoever. I won't trade it. I won't be long of it. I won't be short of it."
While he said he understands "the rationale and the wisdom of the blockchain," the online ledger technology underlying bitcoin, Gartman said it makes no sense to him to have "some sort of an asset that has no real asset value behind it."