The 29-year-old singer and actor gave a military salute to tearful fans Tuesday before walking into a boot camp in the city of Uijeongbu north of Seoul.
Many of the hundreds of fans gathered in front of the base were from Japan and China.
All able-bodied South Korean men are required to serve two to three years in the military, a legacy of the Korean War of the 1950s. Rain isn't expected to receive any special treatment during his time in the army.
Rain's real name is Jung Ji-hoon. He is one of Asia's most popular stars.
He made his Hollywood film debut in "Speed Racer" in 2008.