더블에스301은 5인조 남성 그룹 SS501(더블에스501)의 멤버인 허용생, 김규종, 김형준(막내)이 결성한 3인조 유닛이다.
The Double S 301 is a three-member unit formed by Heo Yongsaeng, Kim Gyujong, and Kim Hyungjun (the youngest member), members of the five-member male group SS501 (Double S 501).