After settling for runner-up in his last two starts, Nakajima finally went one better as he was crowned the winner of the ASO Iizuka Challenged Golf Tournament after a dramatic finale on Sunday.
The 22-year-old Nakajima beat Takumi Kanaya, 25, with a birdie on the second playoff hole after overcoming a three-shot deficit at the start of the final round.
Nakajima closed with a seven-under-par 65 at the Aso Iizuka Golf Club to match Kanaya at 29-under-par 259.
Kanaya, who led the first three rounds, finished with a bogey-free 68.
Nakajima was over the moon after bagging the second JGTO title of his career, but his first as a professional after joining the play-for-play ranks last year.
He was an amateur when he secured his maiden Tour win at the 2021 Panasonic Open.
"I feel so happy. Winning in the playoff against Kanaya-san after a fiercely-battled final day made the victory even sweeter. It was a feeling that can't be expressed in words, " said Nakajima, who came in runner-up at the Mizuno Open before finishing joint second again at last week's BMW JGT Championship Mori Building Cup.
"No, it's no different (than winning as an amateur). The outcome of the game doesn't change, and I believe that the person who plays good golf will win, so it doesn't change.
"After turning professional, I didn't have a good performance for a while, and I saw players of my generation had won it before me.
"But I didn't feel anxious at that time, and I believed that if I continued with my own golf, I could win. I was able to do it with the support and understanding from my family, trainers, coaches, and support staff."
In regulation, a brilliant front-nine display saw Nakajima cancel out Kanaya's three-shot advantage to ensure their two-horse race go down the wire.
Nakajima eagled the par-five opening hole and notched three more birdies on hole numbers three, seven and nine to draw level with Kanaya at 29-under-par.
Kanaya had early birdies on the first and third but could not make any headway.
Nakajima finally overtook Kanaya at the summit for the first time after making another gain on 12, but it wasn't too long before the latter secured his long-awaited third birdie on 14 to put the contest back all square.
Nakajima quickly restored the lead when he birdied the 15th - but he would surrender his pole position once more after bogeying the 17th, the first shot he dropped for the week.
Kanaya capitalised on his playing partner's misfortune to return to the top of the leaderboard.
Despite the late setback, Nakajima wasn't going to give in just yet. As Kanaya settled for par at the last. Nakajima forced a shootout by rolling in a clutch birdie putt from 13 feet.
"It (the final day) felt long, and I was excited and thrilled. I was fired up all day long to contend for the title," he said.
"I was prepared for a close battle, and I think I managed to put my heart into each swing, one swing at a time."
Kanaya was philosophical after the defeat ended his hopes of attaining his career's first back-to-back victories.
"To finish the four-day day tournament with 29-under-par, I'm quite satisfied with my overall performance. There's nothing more I can say," said Kanaya.
"Nakajima-san turned in a wonderful performance today. He played so well in the playoff, so credit to him and he truly deserved the victory."
Leading final round scores:
259: Takumi Kanaya 62-65-64-68, Keita Nakajima 67-64-63-65;
266: Ryo Okamura 66-66-68-66;
268: Takahiro Hataji 65-72-66-65;
269: Kensei Hirata 69-66-65-69;
270: Kazuya Koura 72-66-66-66, Ryosuke Kinoshita 65-69-67-69, Yosuke Tsukada 68-68-64-70;
271: Shunya Takeyasu 70-64-69-68, Taichi Nabetani 67-66-70-68, Yuki Inamori 71-64-67-69, Mikiya Akutsu 64-69-67-71, Yujiro Ohori 65-68-67-71.