The forward joins United from Barcelona, after winning the Primera División de la Liga de Futbol Femenino, Supercopa de Espana Femenina and UEFA Women's Champions League with the Spanish side last season.
On the international stage, Geyse and her Brazil side are current Copa America Femenina champions – after beating Columbia 1-0 in the 2022 final.
Upon signing, Geyse expressed her joy.
"I am very happy to be here," she said. "Signing for United has been a very special day for me and my family.
"I am very grateful for this opportunity and thank everyone for making it happen.