Park Jae-hyung (Korean: 박제형; born September 15, 1992), known professionally as eaJ (제이), is an Argentine-born American singer, songwriter, and former member of the South Korean rock band Day6. He gained early recognition by competing in the first season of K-pop Star in 2012, where he finished in sixth place before signing with JYP Entertainment. Park debuted as the lead guitarist and vocalist of Day6 in September 2015 and was with the group until his departure in December 2021.

In addition to his work with Day6, Park pursued a solo career under the stage name eaJ, launching the eaJ Project in 2020, where he released several self-written tracks on YouTube and SoundCloud. He has collaborated with various artists, including Seori, Keshi, and DPR Live, and was the co-host of the award-winning How Did I Get Here? podcast. Park is also known for his work as an MC on After School Club and his appearances at Head in the Clouds festivals.

As a mental health advocate, Park has spoken openly about his struggles with panic disorder and created the From Friends project, which raised $100,000 for the Jed Foundation to provide free mental health resources. He continues his solo music career, releasing multiple EPs, including Laughing In Insomnia (2023) and Medicated Insomnia (2023).