Kim is best friends with fellow model-actor and School 2013 co-star Lee Jong-suk, whom he has known since their modeling days.
Since May 2015, Kim has been in a relationship with model-actress Shin Min-a.
On May 24, 2017, Kim was diagnosed with nasopharyngeal cancer. Kim's agency, Sidus HQ, said Kim had begun drug and radiation treatment and would halt all activities. On December 29, Kim announced, in a letter, that he had completed his treatment plan for the cancer. However, he was still exempted from military service.