Yasuhiro Suzuki has apologised for putting Seiji Komatsu under "unwarranted suspicion" by tampering with his drink, which caused the latter to fail a drugs test at a national meet in September.
A probe from the Japan Canoe Federation (JCF) also found that Suzuki had made multiple attempts to sabotage several of his team mates, sometimes by stealing their equipment.
"Instead of working hard, I committed misconduct as an athlete and, further, as a member of society," Suzuki said in a statement released by his lawyer.
Suzuki, who has been handed an eight-year suspension by the country's anti-doping agency, faces a life ban from the JCF following the incident.
Komatsu was provisionally suspended after failing a drugs test but the 25-year-old has been allowed to resume his bid to qualify for the 2020 Tokyo Olympics after Suzuki's confession during the JCF probe.
"At first I couldn't believe this kind of thing would happen in Japan," Komatsu said, revealing he had effectively given up on his Tokyo 2020 dreams.
"Until Mr Suzuki confessed, I was in a bad mental state. I began to feel hopeless about (competing at) the Tokyo Olympics, that it was impossible."
Ishikawa prefecture police are also probing the incident following a complaint from Komatsu.