What Western TV series aired the longest in the U.S.?

"Gunsmoke" is an American television western that aired on CBS for 20 seasons (1955–75), becoming the longest-running prime-time television western in history. The series was the top-rated show from 1957 to 1961 and maintained excellent ratings throughout its run. Gunsmoke was set in Dodge City, Kansas, in the 1890s and began as a radio program that CBS aired from 1952 to 1961. Gunsmoke’s success prompted many imitations, but none enjoyed its longevity or rose to its heights. After the series concluded, CBS produced several made-for-TV movies based on the show, including "Gunsmoke: Return to Dodge" (1987), "Gunsmoke: The Last Apache" (1990), and "Gunsmoke: One Man’s Justice" (1994). Arness starred in all of them.

Source: Encyclopedia Britannica