Jay Dee Penn (Kailua, December 13, 1978), better known as B.J. Penn, is a former fighter of mixed martial arts from US. He fought for almost his entire career in the prestigious UFC organization, in which was, after Randy Couture, the second wrestler in the history of the promotion to become champion of two different weight classes, having been welterweight champion in 2004 and champion of the weights light between 2008 and 2010; Also in 2009 by lightweight champion in office he tried to win the welterweight title to become the first fighter in history to simultaneously hold two UFC belts, enterprise never even attempted previously by other fighter. In 2015 he was introduced in the Hall of Fame UFC under the category of the Modern Era.
In 2016, after 18 months of "withdrawal", Penn decided to return to work, again for the US promotion, in the featherweight category. Penn was the first non-Brazilian to win the world championships in Brazilian jiu jitsu black belt category. He is known for his willingness to fight anyone in spite of the weight class they belong, also struggling as when faced light heavyweight Lyoto Machida in 2005. Penn has also participated as a coach at the fifth and at the 19th season of the reality show The Ultimate Fighter. He retired in 2014 after 13-year pro career.