Is it true? Yes! Lee Dong Gun's agency immediately addressed the report confirming, "After checking with the actor, they say they're not quite in a serious relationship yet, but that they have feelings for each other and are getting to know one another. It's been about a month since they've been spending time together."
A rep from Jiyeon's side also commented, "She and Lee Dong Gun are currently dating, and they got close through filming together."
Congratulations to the two!