The NTT Docomo Red Hurricanes were demoted following a 20-17 loss to the Hino Red Dolphins in Saturday's Top League relegation-promotion playoffs.
Shigenori Someyama kicked two conversions and two penalties, the final one breaking a 17-17 tie in the 73rd minute that sent the Red Dolphins into the Top League for the first time.
The Red Hurricanes crossed the whitewash three times, but Riaan Viljoen was unable to convert once, allowing the Top Challenge side to squeak past at Yanmar Stadium Nagai.
"Everyone made this a reality with their desire," said Tadashi Hosoya, the Red Dolphins' director of rugby.
Hino appeared to have opened the scoring with a 12th-minute try that was disallowed by the officials.
But the challengers didn't panic and Someyama opened the scoring with his boot, slotting over a 23rd-minute penalty.
"If you let the Top League team score first, they'll be in the driver's seat," Hosoya said. "Our players succeeded by attacking right from the start."
At Kitakyushu's Mikuni World Stadium, Fukuoka-based Coca-Cola Red Sparks retained their Top League status by the narrowest margin, a 27-27 draw with the Mitsubishi Sagamihara Dynaboars. The Red Sparks scored three late tries to overcome a 24-10 deficit.
In the night cap of the Kitakyushu doubleheader, Munakata Sanix Blues won a Kyushu derby, beating Kyuden Voltex 40-21 with Ahio Siliva and Andre Esterhuizen each scoring a brace to keep the Blues in the Top League for another season.