The fee exceeds the previous record of £685,000, when Bay FC signed Racheal Kundananji last February.
Chelsea set the British transfer record in signing Mayra Ramirez for £384,000 in January 2024.
Girma is Chelsea’s first signing of the January transfer window after they fought off competition from Lyon.
Chelsea made her a priority target after losing defender Kadeisha Buchanan to a season-ending knee injury.
Head of women’s football Paul Green said: “Naomi is a world-class defender who is now coming into the prime years of her career.
“Her composure on the ball, ability to read the game, coupled with her pace and athleticism, will add top quality to an already very talented squad.
“She is undoubtedly ready for this next step in her career and we are delighted it will be with Chelsea.”
In 2023 Girma helped San Diego win the National Women’s Soccer League (NWSL) Shield and was named US Soccer’s female player of the year.
She was part of the USA side that won gold at the 2024 Olympics in Paris.
