Okusa, 23, is best known for his exploits in SUPER GT's GT300 division, where he races for the Gainer Nissan team.
It will be his first outing in a single-seater since he drove in the Formula Regional Japanese Championship on the support bill of last year's World Endurance Championship round at Fuji.
Okusa commented that he was "deeply grateful" for the chance to race at Suzuka, adding: "Friday's FP1 session will be the first time I drive a Super Formula car, but I have confidence in myself based on everything I have done up until now.
"Together with the team, I'll fight to the very end with all my strength to get the best possible results."
The news follows TGM's announcement on Tuesday that Oyu, who joined the rebranded squad from Nakajima Racing this year, had decided to give up his drive for the final two rounds of the season.
It's believed financial factors have played a role in Oyu's decision to stand down from the TGM drive, as well as the fact he stands no realistic chance of racing for the Servus Japan-run squad in 2024.
Okusa has links to TGM via sponsor Ponos, which backs the #10 Nissan GT-R NISMO GT3 he drives in SUPER GT alongside his mentor and ex-Super Formula driver Hironobu Yasuda.
He is also familiar to the Servus Japan organisation, which operates the 'Sutekina Racing' outfit with which Okusa has raced in Formula Regional.
Okusa's experience in single-seater machinery is limited as he has spent much of his career in either Super Taikyu or SUPER GT.
However, he did make a one-off outing in All-Japan Formula 3 in the 2018 season finale at Fuji at the age of 18, and has three wins in Formula Regional under his belt across three seasons of sporadic outings.
Okusa made his SUPER GT debut last year and immediately impressed sharing Gainer's #11 car with Ryuichiro Tomita, staying in contention for the title until the last round and ultimately finishing third overall.
Nissan offered him the chance to sample its Z GT500 car in its annual 'shootout' test at Fuji off the back of that success.
This year has proven tougher for Okusa, who along with Yasuda sits 15th in the GT300 standings with a best finish of fifth, a legacy of the GT-R being handed a tougher Balance of Performance.
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