John Ross "J.R." Ewing Jr. is a fictional character in the American television series Dallas (1978–1991) and its spin-offs, including the revived Dallas series (2012–2014). The character was portrayed by Larry Hagman from the series premiere in 1978 until his death in late 2012, and Hagman was the only actor who appeared in all 357 episodes of the original series. As the show's most famous character, J.R. has been central to many of the series' biggest storylines. He is depicted as a covetous, egocentric, manipulative and amoral oil baron with psychopathic tendencies, who is constantly plotting subterfuges to plunder the wealth of his foes. In the PBS series Pioneers of Television, Hagman claimed the character of J.R. began its development when he played a similar character in the film Stardust, and that he was also inspired by a mean boss he once had.

The focus of the series was initially the feuding families, with J.R. just a supporting character, but his popularity grew and the producers acknowledged he became the breakout character. Two highly rated 1980 episodes became part of a cultural phenomenon that year known as "Who shot J.R.?". In "A House Divided", the audience witnessed J.R. being shot by an unknown assailant.

After the cliffhanger was broadcast in March, the audience had to wait until the November conclusion, "Who Done It". With his new-found popularity, Hagman threatened to leave the series unless his contractual demands were met. CBS leaked rumors of recasting, but the actor eventually prevailed. He also appeared in five episodes of spin-off series Knots Landing between 1980 and 1982. The character is featured in the first two seasons of the 2012 reincarnation series as well. Hagman died on November 23, 2012, and Dallas producers subsequently announced that J.R. would be killed off in the second season. The episode "The Furious and the Fast" dealt with J.R.'s death.

J.R. Ewing is considered one of television's most popular characters, with TV Guide naming him #1 in their 2013 list of The 60 Nastiest Villains of All Time. In 2016, Rolling Stone ranked him #11 of their "40 Greatest TV Villains of All Time". In a 2001 Channel 4 poll in the U.K. he was ranked 38th on their list of the 100 Greatest TV Characters.