The Buick Master Six, also denoted Series 40 and Series 50 based on the wheelbase used, was an automobile built by Buick from 1925 to 1928 and shared the GM B platform with the Oldsmobile Model 30. Previously, the company manufactured the Buick Six that used the overhead valve six-cylinder 242 cu in (4.0 L) engine in their high-end cars, and the Buick Four for smaller, less-expensive cars. After 1924, they dropped the four-cylinder engine and designed a small six, which they called the Buick Standard Six, to replace that end of the market. They coined the name "Master Six" for the high-end cars, now powered by the 255 cu in (4.2 L) engine released the year before. The yearly changes were a result of a new business philosophy called planned obsolescence.

Buick is a division of the American automobile manufacturer General Motors (GM). Started by automotive pioneer David Dunbar Buick in 1899, it was among the first American automobile brands and was the company that established General Motors in 1908. Before the establishment of General Motors, GM founder William C. Durant had served as Buick's general manager and major investor. With the demise of Oldsmobile in 2004, Buick became the oldest surviving American carmaker.