Part of his test was a game against future women's professional Sayuri Takebe, who was member of the apprentice school at the time.

[3] Watanabe passed the entrance exam and entered the apprentice school as a 6-kyū protegee of shogi professional Kazuharu Shoshi.

[1] Watanabe was promoted to the rank of professional 4-dan on April 1, 2000, at the age of 15 after winning the 26th 3-dan league (October 1999 to March 2000) with a record of 13 wins and 5 losses, thus becoming the fourth junior high school student after Hifumi Katō, Kōji Tanigawa and Yoshiharu Habu to become a shogi professional.

[2][7] In 2005, Watanabe defended his Ryūō title for the first time when he defeated challenger Kazuki Kimura 4 games to 0.

As a result, Watanabe became the youngest person ever to be promoted to the rank of 9-dan at the age of twenty-one years and seven months.

[13] Watanabe successfully defended his Kiō title in 2014 against Hiroyuki Miura,[14] in 2015 against Habu,[15] in 2016 against Amahiko Satō[16] and in 2017 against Shōta Chida.

[17] Watanabe's victory over Chida was his fifth consecutive over all, making him only the second player ever to qualify for the Lifetime Kiō title.

[18] In 2012–2013, Watanabe won the 62nd Ōshō Challenger League tournament with a record of 6 wins and 0 losses to advance to the title match against Yasumitsu Satō.

[22] In November 2018, Watanabe defeated Tatsuya Sugai to win the 39th JT Nihon Series [ja].

[23] The following month he defeated Tetsurō Itodani in single game playoff to earn the right to challenge Kubo for the 68th Ōshō title.

[32] In June – July 2020, Watanabe defended his Kisei title against Sōta Fujii, but lost the match 3 games to 1.

[37] In June – July 2021, Watanabe and Fujii met for the second consecutive year in Kisei title, with Watababe being the challenger this time.

[38] The defeat also was the first time in a major titles match that Watanabe was unable to win a single game.

The couple met when Ina was an apprentice female shogi professional and became friends due to their shared interest in tsumeshogi.

[76][77][78] Watanabe is a fan of horse racing as well as professional soccer, even traveling to overseas venues to attend games.