Russell, 80, left the event at the Hyatt Regency resort in Incline Village on a stretcher and was taken to a hospital as a precaution, according to the Celtics.
The Hall of Fame center is "feeling better," the Celtics said, and is expected to fly back to his Seattle-area home as early as Thursday night.
Witness Michael Rooney told The Associated Press that Russell was telling a story about Wilt Chamberlain to a roomful of Konica Minolta employees when he started to sway at the podium and then fell backward. Rooney says Russell seemed to be conscious as hotel personnel and first responders came to his aid. The meeting was cut short and attendees left.
Russell, a five-time NBA MVP, is considered one of the best players in basketball history. He won 11 championships in 13 years as a center with the Celtics. As a player-coach for the last two, he was the first black head coach in a major U.S. pro sport.
Russell was inducted into the Hall of Fame in 1975, was awarded the Presidential Medal of Freedom in 2010, and was honored last year with a statue outside Boston's City Hall.
He was recently at the NBA Finals, awarding the trophy in his name [given to the Finals MVP] to the Spurs' Kawhi Leonard following Game 5 in San Antonio.
ESPNBoston.com's Chris Forsberg and The Associated Press contributed to this report.