Singer Jimmy Buffett and billionaire William “Beau” Wrigley Jr. are getting behind a U.S. marijuana startup, joining a scrum of companies betting on the increasingly crowded cannabis industry.

Mr. Buffett, whose hits like “Margaritaville” evoke a beach-bum lifestyle, will license his Coral Reefer brand to Surterra Holdings Inc. for a line of cannabis products including vape pens, gel caps, edibles and lotions.

The...