Casino operators and investors are making bets that online gambling is ready to take off in the U.S. as the coronavirus pandemic keeps gamblers away from slot machines and blackjack tables.

Casinos have long viewed online gambling warily, as a potential siphon of the dollars that come from gamblers attending their venues. But with bricks-and-mortar gambling decimated by the pandemic, casino operators are treating online gambling as an opportunity, making big investments and pursuing public spinoffs as a way to unlock the value...