Jeong Yun-o (Korean: 정윤오; born Jeong Jae-hyun (정재현), February 14, 1997), known professionally as Jaehyun, is a South Korean singer. He is a member of the South Korean boy band NCT and its sub-units NCT 127 and NCT DoJaeJung. He made his solo debut on August 26, 2024, with the studio album J.

First introduced in December 2013 as a trainee under the pre-debut team SM Rookies, he debuted in April 2016 as a member of the rotational unit NCT U and as a member of Seoul-based fixed unit NCT 127 in July 2016. He later joined the fixed unit NCT DoJaeJung in April 2023.

As of 2024, Jaehyun has released five solo singles: "Forever Only" and "Horizon", both as part of the NCT Lab project, while "Roses", "Dandelion", and "Smoke" as part of his debut album J. Apart from music, he has also hosted the radio show NCT Night Night from March 2017 to January 2019 and the television program Inkigayo from October 2019 to February 2021. He later debuted as an actor with a leading role in the college romance television series Dear. M (2022), and he is set to make his feature film debut in the mystery thriller film You Will Die In 6 Hours. As of June 2022, Jaehyun is a global ambassador for the Italian luxury brand Prada; he is considered among the most impactful fashion ambassadors from South Korea.

Jaehyun was born on February 14, 1997, in Seocho District, Seoul, South Korea. Although he was born with the name Jeong Jae-hyun (정재현), he later legally changed his name to Jeong Yun-o (정윤오). At the age of five, Jaehyun moved to Connecticut, USA with his parents, where he went by the English name Jay Jeong. As a result, he is fluent in both English and Korean. He revealed that while studying in Connecticut, he was the only Asian student at school. Jaehyun returned to South Korea at the age of ten, and said that he struggled adapting back to Korean culture.

Jaehyun trained in Taekwondo and received a Black belt. He later became interested in music at a young age due to his parents' influence; he learned to play the guitar and the piano, and eventually began singing during his middle school. His grandmother, a retired professional dancer, was also his guide during dance training. He was also inspired to pursue music by his elementary school teacher, Lee Eun-yeol. Jaehyun attended Shindong Middle School and Apgujeong High School and later transferred to the School of Performing Arts Seoul in his second year of high school and graduated in 2016. He used to be a student council president.

Jaehyun was cast by SM Entertainment in 2012, discovered by street scouts in front of his school. He auditioned with a performance of "Mama" by Exo. On December 9, 2013, he was introduced as a member of the pre-debut trainee team SM Rookies. In 2014, he participated with fellow NCT members on the show Exo 90:2014, where they performed songs from the 1990s. From January 21 to July 1, 2015, he and groupmate Doyoung were hosts of the program Show Champion. In July 2015, he and groupmate Ten both competed in Hope Basketball All-Star. In December 2015, Jaehyun performed "A Whole New World" on Disney Channel Korea's The Mickey Mouse Club.

In April 2016, Jaehyun officially debuted as a member of NCT's sub-unit, NCT U, with the singles "The 7th Sense" and "Without You". On July 10, he made his debut as a member of NCT 127 with their first EP, NCT#127. In March 2017, Jaehyun joined the radio show NCT Night Night as a DJ alongside fellow member Johnny. In November 2017, Jaehyun and Dear released the song "Try Again" for the project SM Station. In December, he featured in the music video for Wendy and Baek A-yeon's "The Little Match Girl". In January 2018, Jaehyun released the single "Timeless" alongside a new NCT U lineup as part of the SM Station project.

In October 2019, Jaehyun became one of three hosts for the South Korean music program Inkigayo alongside Minhyuk and Naeun, hosting the show every Sunday. On September 4, 2020, it was confirmed that Jaehyun would make his acting debut as Cha Min-ho – a second-year computer science student at Seoyeon University, in the KBS2 drama Dear. M, which was to be released in 2021. The airing of the drama was postponed indefinitely after lead actress Park Hye-su was accused of school bullying. After a nearly two-year delay, the first six episodes of Dear. M were released globally on June 29, 2022, through the streaming service Viki. Its following episodes were released on July 6, 2022.