Google's original restrictions, which it announced in March and rolled out in June, were intended to protect consumers and included initial coin offerings (ICOs), wallets and trading advice, which are still not allowed.
While the cryptocurrency boom has produced both excitement and wealth, it has also spawned fraud and high-profile scams, as regulation struggles to catch up with a fast-moving space. At various points earlier this year, Google, Facebook, Twitter and Snap all cracked down on crypto-related advertising to stop bad actors, though their initial hard-line approaches prevented even legitimate businesses from buying ads.
"We don't have a crystal ball to know where the future is going to go with cryptocurrencies, but we've seen enough consumer harm or potential for consumer harm that it's an area that we want to approach with extreme caution," Google's Scott Spencer told CNBC at the time of its original ban.
Google's updated policy applies to advertisers all over the world, though the ads can only run in the U.S. and Japan, and interested parties will need to apply for certification to serve ads in each country individually. Google's move follows Facebook, which started allowing preapproved cryptocurrency advertisers in June.
Google parent company Alphabet gets roughly 86 percent of its total revenue from advertising. The company booked more than $54 billion in ad revenue in the first half of 2018.