The 1975 Houston Astros season was a season in American baseball. The team finished last in the National League West with a record of 64–97, 431⁄2 games behind the Cincinnati Reds. The Astros' .398 winning percentage is, as of 2021, the fourth worst in franchise history. In three consecutive seasons, in 2011, 2012 and 2013 the Astros finished each season with lower winning percentages.^[1] Houston also lost 97 games in 1965 and 1991, but lost more than 100 games each in the 2011-2013 seasons. This was the first season the Astros donned their now famous rainbow uniforms. The uniforms would make slight alterations throughout the years. In 1975, the numbers on the back of the jersey were inside a white circle but by the following season, the white circle was eliminated entirely. In 1987, the rainbows were relegated to the shoulders of both home and away uniforms (prior to that they were only used on the away uniforms shoulders) and by 1994, the rainbow uniforms were retired after 19 years for a more contemporary look.

• December 3, 1974: Lee May and Jay Schlueter were traded by the Astros to the Baltimore Orioles for Enos Cabell and Rob Andrews.^[2]
• January 5, 1975: Pitcher Don Wilson died of carbon monoxide poisoning.