It's only the latest in a string of casino closures in a city facing increased competition from Las Vegas megaresorts and tribal casinos in California.
The Sundowner closure affects 300 full-time and 75 part-time employees, who learned about it in a letter from owners.
The resort, which opened in 1975, has been for sale for several years.
Employees, who also were notified that their health insurance will cease on October 17th, say they expected the closure.
Other downtown casinos closing in recent years have included the Flamingo Reno, Comstock, Pioneer and the Riverboat. The Flamingo Reno later reopened as the Golden Phoenix Hotel Casino.
There are no plans for the Sundowner building.