Rybakina, who has won two WTA 500 titles this season, beat Canada's Victoria Mboko 6-3 7-6(4) in the Tokyo quarters.
The 26-year-old will be making her third consecutive appearance at the Riyadh showpiece, which runs from November 1-8. Russians Mirra Andreeva, 18, and Ekaterina Alexandrova, 30, were named as the first and second alternates.
Former Wimbledon champion Rybakina joins world number one Aryna Sabalenka, Iga Swiatek, defending champion Coco Gauff, Amanda Anisimova, Jessica Pegula, Madison Keys and Jasmine Paolini in the draw.
"This season we have seen eight different champions across WTA 1000 tournaments and the Grand Slams, showing the incredible depth and drama of the WTA Tour, with the perfect curtain-call now ready to take place at the WTA Finals," WTA Finals Tournament Director Garbine Muguruz said in a statement.
Rybakina, who will rise one spot to world number six when the latest rankings are released on Monday, faces Czech Linda Noskova in the Toyko semi-finals on Saturday.