The announcement was made on Friday by Toyota and his Cerumo team, for which the 48-year-old has driven continuously since the 1999 season.
In that time, he scored three GT500 titles in 2001, '05 and '13 while racking up 19 wins, putting him second only to Tsugio Matsuda in the all-time list, and 24 pole positions, an outright series record.
In a short statement, Tachikawa said: "It has been with the help of Toyota, TCD [Toyota Customising and Development], Cerumo, [long-time team title sponsor] ZENT and Bridgestone, as well as the support of many friends and family, that I have been able to come this far.
"I want to thank you all for respecting my feelings until the end. My heart is full of gratitude.
"The GTA has always provided a wonderful stage known as SUPER GT, and being able to compete on this stage has been a blessing for me.
"I decided to make this announcement in the middle of the season so that those who have been supporting me have the chance to see my last races properly. I will give it my all for the remaining races, so please support me until the end."
Tachikawa scored his first GT500 title in only his third full season, sharing a first-generation Supra with Hironori Takeuchi. Four years later he earned a second crown, this time paired with ex-Formula 1 racer Toranosuke Takagi.
In 2011, he was joined at Cerumo by Kohei Hirate, and the duo would claim the final title for the Lexus SC430 two years later.
Tachikawa paired up with current Cerumo team-mate Hiroaki Ishiura in 2015, but the pair have struggled for form since the switch to the GR Supra in 2020, scoring only two podiums in that time. Their last win came at Fuji in 2019.
Toyota Gazoo Racing President Tomoya Takahashi added: "Tachikawa has been racing as a Cerumo GT500 driver since 1999, and has given us some brilliant results, such as his three championship titles.
"In addition, he has been involved in the development of successive GT500 cars like the Supra, SC430 and LC500 for a long time, and has always contributed to creating cars that are capable of fighting for victory.
"I think it has been challenging to continue representing Toyota as a driver and to support Toyota's motorsport activities, so I have nothing but gratitude for your distinguished service.
"I also want to express my heartfelt thanks to the family that has supported and worked hard alongside Tachikawa. Thank you for being involved in GT500 for such a long time. That said, the season is not over, so let's keep pushing together until the end."
Outside of SUPER GT, Tachikawa was a regular presence in Formula Nippon (now Super Formula) from 1997 to 2009, and also made a single appearance in the Le Mans 24 Hours in 2008, driving for Dome.
He has served as team director for the Inging Super Formula team, which is run jointly by Cerumo, since 2011.
Tachikawa's announcement comes just days after he tested the 2024 version of the Toyota GR Supra in this week's shakedown test at Okayama. Friday's statement made no mention of any future roles at Cerumo or Toyota.
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