Harada won the Japanese 125cc Junior championship in 1988 before moving to the 250cc category, where he finished as runner-up to Tadayuki Okada in the All-Japan 250cc series in both 1990 and 1991. He claimed the domestic title in 1992 and made wildcard appearances at the Japanese round of the 250cc World Championship across those three years, twice starting from the front row and twice scoring points.
Riding a Yamaha TZ250 in his first full season of world championship competition in 1993, Harada won four races including his home event and claimed the 250cc world title — defeating Honda's Loris Capirossi. The achievement was all the more notable for coming in a debut campaign at that level.
A wrist injury hampered his 1994 defence, limiting him to seventh overall with a single podium finish. In 1995 Harada emerged as Max Biaggi's primary rival for the 250 championship, winning one race and finishing second eight times, but Biaggi retained the title. His 1996 season was constrained by underpowered machinery; he reached the podium four times and was contemplating retirement when Aprilia offered him a works ride for 1997. That season he accumulated 235 points — his highest seasonal total — but still finished third behind Biaggi and Ralf Waldmann.
The 1998 250cc season produced one of the most contested championship finales in Grand Prix history. Harada, again with Aprilia, led the championship for the majority of the year over teammate Capirossi. At the final round in Argentina, Harada held the lead into the last corner of the last lap with the world title within reach. Capirossi's machine struck the rear of Harada's, sending the Japanese rider off the track. Valentino Rossi took the victory and Capirossi claimed second, which was sufficient for Capirossi to secure the championship. Harada finished the season third behind Capirossi and Rossi. The incident, widely discussed in the paddock, remained controversial.
In 1999 Harada stepped up to the 500cc class aboard Aprilia's 380cc V-twin machine. He produced competitive results in the early part of the season, including podium finishes at Paul Ricard and Donington Park, and scored top-five finishes in the first ten races. However, the bike's performance declined in the second half of the year and he could not sustain that form. In 2000 he was less competitive overall, finishing sixteenth in the championship.
In 2001, Harada returned to the 250cc class in an attempt to win a second world championship. He took eight pole positions and three race victories, but the title went to fellow Japanese rider Daijiro Kato. For 2002, Harada entered the new MotoGP class with a Honda two-stroke machine, but the season was difficult and he finished seventeenth overall. He retired from professional racing at the end of that year.
Tetsuya Harada's career is defined by the 1993 250cc world championship, claimed in his first full season of Grand Prix competition, and by the events at the 1998 Argentine Grand Prix — a finale that denied him a second title in circumstances that remain a reference point in discussions of racing ethics and championship climaxes. His 17 Grand Prix victories across a decade of 250cc competition place him among the significant figures of that class during one of its most competitive eras.