Han So Hee and Ryu Jun Yeol were spotted together in Hawaii.
A fan posted their photos online and sparked dating rumours.
The idols' respective agencies initially denied any romantic involvement.
Reportedly, an online feud erupted between former Girls' Day member Hyeri and Han So Hee.
Previously, Hyeri and Ryu Jun Yeol were together for seven years.
A day after the denial, Han So Hee confirmed that she was dating Ryu Jun Yeol.
Han So Hee, in a statement, promised to apologise to Hyeri.
Han So Hee clarified that she began dating Ryu only after he ended his relationship with Hyeri.
Later, Korean media outlet also shared photos of the couple on a date in Hawaii.
However, Ryu Jun Yeol has yet to comment on the recent events.