"Bad Day" is a pop song by Canadian singer-songwriter Daniel Powter from his self-titled second studio album (2005). It was written by Powter and produced by Jeff Dawson and Mitchell Froom. Powter and Dawson recorded the song in 2002, but were initially unable to find a record label to release it. The song was first used in a French Coca-Cola television advertisement in Christmas 2004 before its official release. Tom Whalley, Warner Bros. Records' chairman and CEO, offered Powter a contract after hearing a demo tape of it. This track ended up being released as the aforementioned album's lead single in Europe in early 2005.

The song was received with a generally mixed reception from critics. While some praised its lyrics for their "universal appeal", others felt they were too broad. Despite this, it was a commercial success. In 2005, the single charted in the top five in more than ten countries worldwide and became the most played song on European radio. After its European success, it was released in the United States where it topped the Billboard Hot 100, Pop 100, Adult Top 40, and Adult Contemporary charts. In 2006, it became the first song ever to sell two million digital copies in the United States, as well as being measured by Billboard as the most popular song of the year inside of the United States, and the first under its new system. After another million were sold, it was certified three-times platinum by the Recording Industry Association of America (RIAA) in 2009. It was certified platinum in Australia, Canada, and the United Kingdom, gold in Denmark and Germany, and also received a certification in France and Japan.

The accompanying music video for "Bad Day" was directed by Marc Webb and became the eighth most-watched music video on the Internet in 2006, reaching 9.8 million views one year after its release. The video depicts two downcast people sharing a similar routine until they meet each other at the end of the video. The song was used for advertisements and television programs, most prominently as American Idol's elimination song. Different shows and artists covered and parodied "Bad Day", including Saturday Night Live and Alvin and the Chipmunks. Powter has also performed the song on television shows including The Tonight Show with Jay Leno and The Ellen DeGeneres Show, and during his concert tours in North America and Europe. The song's success made it Powter's "anthem" and would be included on his later compilation albums B-Sides (2007) and Best of Me (2010).