Veterinarians and SeaWorld staff spent three days giving Kyara critical care, but officials said her health continued to decline.
Theme park officials said Kyara died early Monday morning from a likely case of pneumonia.
LOOKING BACK: Kyara last killer whale born in captivity at SeaWorld
A post-mortem examination has been scheduled to determine the calf's official cause of death.
"Kyara had a tremendous impact on each of her care staff, not to mention all of the guests that had the chance to see her," SeaWorld said in a statement. "From late nights to early morning, rain or shine, we dedicate our lives to these animals, and this loss will be felt throughout the entire SeaWorld family."
SeaWorld said it is monitoring the health of its other killer whales for any signs of illness.
Kyara was born in April to mom Takara.