The design, synthesis, and applications of new boranes are eternal topics in boron chemistry. A new bis(borane)alkanethiolate salt, K[BH3S(CH3)BH3], was synthesized in high yield by the reaction of K with (CH3)2S·BH3 at room temperature. The formation mechanism was elucidated based on experimental and theoretical studies. The single-crystal structure of the K[BH3S(CH3)BH3]·18-crown-6 adduct was determined in which the B-S-B bonding information of K[BH3S(CH3)BH3] was illustrated for the first time. Using K[BH3S(CH3)BH3] as a starting material, KB3H8 was successfully synthesized.