Watanabe, who campaigned only in Japan and South Korea, was one of the first world Jr. Bantamweight champions, as the division was relatively new when he was crowned.
He started his professional boxing career with a three round knockout over Keiza Miyazaki. The fight was in Okayama, Watanabe's birthplace. Two first round knockout wins followed, one over Yukihiro Kawahira, and another over Noburu Iishi. There was an immediate rematch with Iishi, and, although the second time around Iishi gave him a tougher test, nevertheless, Watanabe still came out a winner, by a knockout in six.
Three more knockouts followed, two in the first round, including one over Koji Kobayashi, brother of former world champion Royal Kobayashi. Then, Watanabe was taken the distance for the first time, against Jin Hyun-Chun in Nagoya, Watanabe winning a six round decision.
After two more decision wins, Watanabe flew to South Korea, where he challenged the WBC world Jr. Bantamweight champion Chul-Ho Kim, losing in his first world title bid by a 15 round decision. Watanabe then returned to Japan and won his remaining four bouts for 1981 there, three by knockout. One of the fighters he beat was Tito Abella, who by then had been ranked as the number one Jr. Bantamweight challenger in the world. Abella was knocked out in four rounds.
On April of 1982, the WBA Jr. Bantamweight champion of the world, Rafael Pedroza of Panama, the cousin of Eusebio Pedroza, travelled to Osaka to defend his belt against Watanabe on April 8. Watanabe won a unanimous 15 round decision and became world champion. Watanabe's remaining fights of 1982 were title defenses against former world champions, Gustavo Ballas of Argentina, knocked out in nine rounds, and Shoji Oguma (former two time world Flyweght champion), beaten by a knockout in 12.
1983 was another busy year for the champion: He beat Luis Ibanez of Venezuela by a knockout in eight, Roberto Ramirez of Mexico by a decision in 15, and Soo-Chun Kwon by a technical decision in 11. Watanabe had built a considerable points lead over Chun Kwon, but in round ten, their heads collided, causing a gap in Watanabe's head. He bled profusely, and the fight doctor ordered the fight to be stopped in round eleven, but since it was ruled that the cut was the product of a headbutt, the fight then went to the scorecards, and Watanabe was declared the winner.
In 1984, Watanabe disposed of another Venezuelan challenger, Celso Chavez, by knocking him out in 15 rounds. Then, he and WBC world champion Payao Pooltarat went to unify the world's Jr. Bantamweight title, and Watanabe finally earned the WBC's belt, winning by a 12 round decision. There was an immediate rematch, and Watanabe again imposed his will, with an 11 round knockout over the former world champ.
In 1985, he retained the title with a 12 round decision over Julio Soto Solano. Wins over Katsuo Katsuma (KO 7) and Suk-Hwan Yun (KO 5) followed, but in 1986, he defended against the late Gilberto Roman in what would turn out to be his last fight. He lost to Roman by a 12 round decision and announced his retirement from professional boxing. Unlike so many others, he was able to stay away from the temptation of coming back and trying to regain his old popularity.
In 1999, Watanabe was spotted at a Seoul activity honoring Asia's greatest boxing champions, alongside Fighting Harada, Jung Koo Chang, Gushiken, Sot Chitalada, and former rivals Ho Kim and Pooltarat, among others.
He had a record of 26 wins, 2 losses and 18 wins by knockout.