"My heart is heavy. Walter Koenig, our Ensign Pavel Chekov, passed away early today," George Takei, who starred alongside Koenig in the show and was a long-time friend, wrote on Facebook on Monday.
Koenig died at his home in Los Angeles, the Associated Press reported citing his talent agent.
Star Trek creator Gene Roddenberry picked Koenig, who had played only a few prior roles on television, when he decided to add a Russian character to the crew of the fictional Starship Enterprise.
The introduction of a Russian officer who worked harmoniously with a racially and culturally diverse crew came amid the real-world Cold War tensions of the era.
"We were stating tacitly that peoples of all races and creeds, nationalities could not only co-exist but care for each other deeply, and in doing so we were heralding a future that was very optimistic," Koenig once told a BBC interviewer.
While Koenig was the son of Russian Jewish immigrants, he used an inauthentic Russian accent that was sometimes played for laughs.
The original TV series ran for three seasons, from 1966 to 1969.
Koenig appeared on 36 episodes, and then in seven movies from 1979 to 1994.
"I was informed while I was still there at my audition that I had the role," Koenig told the Joplin Globe in 2008.
"It seemed like an engagement that might work out over several episodes, but I had no idea that it would totally shape my career and my life."
The show remained popular in syndicated reruns and inspired several later TV series and a succession of films.
It attracted a devoted worldwide fan base and earned an enduring place in pop culture.
Koenig's Star Trek fame was a blessing and a curse, as it was for cast mates such as William Shatner (Captain Kirk), Leonard Nimoy (Spock), DeForest Kelley (Dr McCoy), Nichelle Nichols (Uhura), James Doohan (Scotty) and Takei, who played Lieutenant Sulu.
They were often typecast because they were so closely associated with Star Trek.
"Most of the '70s, I didn't work at all, and I blamed it on typecasting," Koenig told the St Petersburg Times in 1998.
He later landed a regular role on the 1990s sci-fi series Babylon 5 and was an author, script writer and occasional director.
