For more than a year, the stars of the hit South Korean military romance Descendants Of The Sun had dismissed rumours that they were dating in real life. Now, they are set to wed on Oct 31.
In a letter published on his fan page on Wednesday, the 31-year-old actor described his 35-year-old co-star as a "precious friend" he made in the course of a project that would change his career. The drama was shot in Greece and South Korea in 2015, before it premiered in February 2016 and swept Asian viewers off their feet.
After the two confirmed their feelings for each other and became a couple, he wrote, "at the beginning of 2017, we made a promise to each other to spend the rest of our lives together", according to allkpop website.
"We'll fill what we each lack with love, and we'll live a new life where we can face difficulties together."
In January, he also bought a two-storey house in the posh neighbourhood of Itaewon-dong, Seoul, which may be the marital home, said The Korea Herald.
In his letter, he said he had not been able to reveal his marriage plans sooner, out of respect for his family.
He added that he did not want to "cause any harm" to his upcoming film, The Battleship Island, according to Yonhap News Agency.
In her note on her fan page, Song Hye Kyo said she felt she and her co-star had similar values, and they stayed in contact as close colleagues and friends after the project was finished.
"The trust he made me feel after a long time and the trust in the future I had with him made me realise his sincere feelings towards me. That's when I became sure."
Rumours of the real-life romance surfaced in March 2016, after they were seen together in New York.
Osen website, quoting a source who is close to the two Songs, said they had begun dating before the show aired. The source said: "In the beginning of last year, Song Joong Ki secretly sent snacks to Song Hye Kyo while she was participating in a photo shoot. This was before Descendants of The Sun aired and the staff members were confused as to why Song Joong Ki was there."
On Thursday, the actor's buddy, Running Man star Lee Kwang Soo, also reacted to the couple's news with comical confusion.
Lee posted on Instagram a photo of a stone on which his friend had written "Joong Ki (hearts) Kwang Soo" in October 2015, during the Descendants shoot in Greece, and asked: "So then what was all this?"