The singer will undergo a four-week basic training in Nonsan, South Chungcheong Province, and then he will be assigned to a police unit, according to Around Us Entertainment.
Lee plans to unveil a two-track digital single album Thursday afternoon to say goodbye to his fans.
"I've been so happy for the past 10 years because of you," he said through a handwritten letter released by the agency. "Now it is the time for us to have a rest. As the past 10 years have passed rapidly, the next one year and seven months will help us feel grateful to each other."
Debuting in 2009, Lee has played a role as the main dancer and a lead vocalist of the five-member boy group BEAST, which was renamed Highlight in 2017.
His teammates Yoon Doo-joon and Yang Yo-seob have been enlisted, and Song Dong-woon will join the military next month.
Yong Jun-hyung left the band earlier this year after being involved in an illegal video scandal. (Yonhap)