SAN FRANCISCO — At a restaurant meeting in California a few years ago, Brad Hirsch and one of his law clients gathered over a meal with two potential business partners: Andrey Kukushkin and Andrey Muraviev, an investor who had flown in from Russia.

Mr. Kukushkin and the Russian financier were hoping Mr. Hirsch could help them build a stake in the state’s burgeoning cannabis market, Mr. Hirsch said, and he helped them set up a real estate business that would cater to marijuana operators. Over the span of just a few years, Mr. Kukushkin would join or develop cannabis companies around San Francisco, Sacramento, Los Angeles and Las Vegas, establishing a foothold in everything from real estate and cultivation to retail and delivery.

There was a reason that people like Mr. Kukushkin, who was born in Ukraine and later worked at a Russian investment bank, had a unique opportunity to get in on the ground floor. Federal law still treats cannabis as an illegal substance, and traditional banks have been wary of getting involved. Wealthy financiers have moved in to fill the void — including a growing cast of investors from Russia and former Soviet Union countries who have helped shape the industry's growth.

One of the nation’s largest cannabis companies, Curaleaf, is led by one of Russia’s most influential financiers and backed by another, allowing the company to pursue rapid expansion and hefty acquisitions. Investment firms have taken their own stakes : A San Francisco-based venture capital fund run by the Russian tech entrepreneur Pavel Cherkashin, backed largely by investors from Russia and the former Soviet Union, has put $2 million into Pure Spectrum, a Colorado-based business marketing CBD products.