Yaegashi finished his amateur career with a record of 56–14 after winning the Inter-High School Championship and the National Sports Festival of Japan. His professional debut was at the Yokohama Cultural Gymnasium in March 2005. He won the vacant OPBF minimumweight title there via a fifth round technical knockout in April 2006 and defended the title once before vacating it. In his first world title shot against the WBC minimumweight champion Eagle Kyowa at the Pacifico Yokohama in June 2007, Yaegashi lost for the first time by a wide points margin after suffering a broken TMJ in two places due to an accidental headbutt in the second round. After a nearly eleven-month absence from the ring, Yaegashi restarted his career, but lost in the semi-final match of the annual Japanese title elimination tournament nicknamed "The Strongest in Korakuen" at the Korakuen Hall in Tokyo in July 2008. It was in June 2009 that Yaegashi won the vacant Japanese minimumweight title at the IMP Hall in Osaka. He defended the title three times before returning it. Yaegashi won the WBA minimumweight title in his second world title shot against Thai's Pornsawan Porpramook via a tenth round technical knockout at the Korakuen Hall on October 24, 2011. It was a very tough fight. Before the final round began, Yaegashi's manager Ohashi said to him "Come back for your son". The fight earned accolades from international media, winning Fight of the Year honors from ESPN.com and BoxingScene.com, as well as the WBA's award for Most Dramatic Fight of 2011. On June 20, 2012, in a match that marked the first time two Japanese fighters had met to unify world titles, he lost to the WBC champion Kazuto Ioka via a unanimous decision at the Bodymaker Colosseum. When asked whether the result would have been different unless he got swollen eyes, Yaegashi said "We should not think about it. There is no if's in boxing". The day after the fight, Ioka decided to move up a weight division. So, Yaegashi's team hope the rematch with Ioka in a higher division as their second unification bout. First, Yaegashi aims to win a world championship again. Yaegashi returned to the ring in a light flyweight bout at the Korakuen Hall on January 5, 2013 to knock out Saenmuangloei Kokietgym in the ninth round. Yaegashi became a two-weight class champion when he defeated defending WBC and lineal flyweight champion, Toshiyuki Igarashi in a unanimous decision on April 8, 2013 in Tokyo, Japan. On 12 August 2013, Yaegashi successfully defended his title by defeating Mexican fighter Oscar Blanquet. Yaegashi floored Blanquet in the 8th round on his way to a points victory. Yaegashi became a three-weight world champion after defeating IBF light flyweight champion Javier Mendoza via Unanimous decision.
