Officials at the Providence Regional Medical Centre said 14-year-old Gia Soriano died on Sunday night.
Another girl was killed during the shooting on Friday at Marysville-Pilchuck High School north of Seattle. The shooter, Jaylen Fryberg, died of a self-inflicted wound.
Three other students remain in hospital, two in critical condition and one in serious condition.
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Parents and students have gathered in a gymnasium at the school on Sunday for a community meeting, with speakers urging support and prayers and tribal members playing drums and singing songs. Fryberg was from a prominent Tulalip Indian family.