Sei (세이) is a South Korean artist under Fantagio Entertainment. She is a member of the Korean Pop girl group Weki Meki.
- Stage Name: Sei (세이)
- Birth Name: Lee Seojeong (이서정)
- Birthdate: January 1, 2000
- Birthplace: Ansan, Gyeonggi, South Korea
- Blood Type: B
- Height: 163cm
- Weight: 49kg
- MBTI: INFJ
- Hobbies: Listening to music
- Specialties: Smiling with her eyes
- She was a trainee for one year prior to her appearance on Produce 101.
- She formerly trained under LOUDers Entertainment.