The 2005 National League Championship Series (NLCS), the second round of the 2005 National League playoffs, matched the Central Division champion and defending league champion St. Louis Cardinals against the wild card qualifier Houston Astros, a rematch of the 2004 NLCS. The Cardinals, by virtue of having the best record in the NL during the 2005 season, had the home-field advantage. The Astros won the series four games to two, and became the National League champions; they faced the American League champion Chicago White Sox in the 2005 World Series, where the Astros lost to the White Sox in a sweep in four games.

The National League Championship Series (NLCS), also known as the National League Pennant, is a best-of-seven playoff and one of two League Championship Series comprising the penultimate round of Major League Baseball's (MLB) postseason. It is contested by the winners of the two National League (NL) Division Series. The winner of the NLCS wins the NL pennant and advances to the World Series, MLB's championship series, to play the winner of the American League's (AL) Championship Series. The NLCS began in 1969 as a best-of-five playoff and used this format until 1985, when it changed to a best-of-seven format.