The former KSW featherweight champion improved to 4-0 under the RIZIN banner with his fourth submission finish, tapping Sasaki with a rear-naked choke in the second stanza after surviving a knockdown in the opening round.
Unbeaten since signing with the promotion, Koike is now hoping to earn a crack at the 145-pound gold against Juntaro Ushiku and join teammate Roberto Satoshi as RIZIN champions.
In the co-main event, one-time Shooto title contender Kazuma Kuramoto scored a violent knockout over Kenji Kato, stopping with knees to the face on the ground late in round one.
The fastest finish of the night came in a featherweight showdown between Sora Yamamoto and Suguru Nii, with Yamamoto stopping his opponent in less than 40 seconds. Yamamoto dropped Nii with a right hand while waking backwards and followed it up with punches and knees until the referee called it.
Check the complete RIZIN Trigger 2 results below.
Kleber Koike def. Ulka Sasaki via submission (rear-naked choke) - Round 2, 3:22
Kazuma Kuramoto def. Kenji Nato via TKO - Round 1, 4:16
Akira Okada def. Takumi Suzuki via unanimous decision
Sora Yamamoto def. Suguru Nii via TKO - Round 1, 0:35
Kohei Tokeshi def. Harry Stallone via KO - Round 2, 3:31
Hikaru Yoshino def. Daisuke Endo via unanimous decision
Kouki Nakagawa def. Katsushi Kojima via unanimous decision
Takuma Uchiyama def. Kotetsu Hara via split decision