Koyama began racing in karts before attending the Formula Toyota Racing School and transitioning to Japanese Formula 4 in 2015. She quickly established herself as a frontrunner in women's motorsport, winning the inaugural Kyojo Cup — an all-women's racing series — in 2017 and defending the title as champion again in 2018. The Kyojo Cup victories drew wider attention to her talent and set the stage for her entry into international competition.
In 2019, Koyama was selected as one of the 18 permanent drivers for the inaugural season of the W Series. At the opening round at Hockenheim she qualified 17th and charged through to seventh place at the finish, also setting the fastest lap of the race. She collected three further points finishes over the remainder of the season to place seventh in the championship standings. After the 2020 season was cancelled due to the COVID-19 pandemic, Koyama returned to the series in 2021, while also making a one-off appearance in the Super Formula Lights championship at Fuji Speedway.
Named a Toyota Gazoo Racing Driver Challenge academy driver for 2022, Koyama contested the full Formula Regional Japanese Championship for Super Licence season with Toyota backing. She dominated the campaign, winning seven races and clinching the series title at Fuji Speedway in September. The victory made her the first woman ever to win an FIA-certified all-gender single-seater championship, a milestone that placed her alongside the most significant firsts in women's motorsport history.
In February 2023, Koyama was announced as the third driver for Anest Iwata Racing with Arnage in the Super GT Series GT300 class, becoming the first woman to race in the championship since Cyndie Allemann in 2012. After passing her rookie orientation test at Fuji Speedway in April, she made her Super GT debut in the fourth round at Fuji, finishing 19th in class. In 2024 she moved to the Arnage-affiliated R'Qs Motor Sports squad, making three appearances in their Mercedes-AMG GT3 Evo. For 2025, Koyama joined apr, driving the Lexus LC500h GT300 alongside Oliver Rasmussen and Yuki Nemoto.
Koyama has been active across multiple series beyond her core Super GT programme. In 2024 she drove an apr Lexus RC F GT3 in the Super Taikyu Series, and at the fifth round at Suzuka she became the second female driver to take an overall victory in Super Taikyu and the first since Kumi Sato in 1993. That same year she competed in Lamborghini Super Trofeo Asia with Iron Lynx Theodore Racing, finishing second in the Pro class standings alongside co-driver Hon Chio Leong after five second-place finishes across the season. At the close of 2024 she also entered the Toyota Gazoo Racing GR Cup North America finale at Indianapolis, participated in Formula E's all-female pre-season test with Lola Yamaha ABT, and took part in the Super Formula post-season winter test at Suzuka with KCMG.
Koyama represents a new generation of Japanese women in professional motorsport. Her 2022 Formula Regional championship victory stands as a benchmark achievement, demonstrating that women can compete for and win open-wheel titles in genuinely mixed-gender fields. Her continued progression into the competitive Super GT Series and her broad participation across GT and single-seater platforms in Asia and beyond mark her as one of the most versatile and determined female drivers to emerge from Japan in the modern era.