The 11th-hour agreement with the Japan Innovation Party (JIP) came just a day before the lower house was due to vote on Takaichi's appointment as the fifth prime minister in as many years.
If she wins, she will take office the same day.
"I'm very much looking forward to working with you on efforts to make Japan's economy stronger, and to reshape Japan as a country that can be responsible for future generations," Takaichi told the JIP's co-head Hirofumi Yoshimura as they signed the deal.
Yoshimura said he was convinced the parties were "on the same page about our passion to move Japan forward".
Takaichi, 64, seen as a China hawk and traditionalist from the right wing of the ruling Liberal Democratic Party (LDP), won the party leadership this month.
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