John Howard Dadley Day (born 24 December 1955) is a West Australian state politician.

Attending Guildford Grammar School, Day later completed a Bachelor of Science, and a Bachelor of Dental Science at the University of Western Australia going on to work at the Perth Dental Hospital as well as other clinical situations including Canning Vale Prison.

He later set up a private practice with his sister Elizabeth working in such places as Wundowie, Glen Forrest and Beverley.

In February 1993, a state election brought the Liberal Party and their coalition partner, the Nationals, to power with a stable majority to form the 34th parliament of W.A.

Led by Premier Colin Barnett, Day held the following ministerial portfolios; He was also the Minister for Environment and Minister for Youth from 22 November to 14 December 2010, having temporarily taken over Donna Faragher's portfolios, and served as Leader of the House for 4 years from 21 March 2013 to 11 March 2017.