Kyara was being dealt with for a disease, likely pneumonia, that SeaWorld's veterinary and creature mind staff was "forcefully treating", as indicated by the announcement.
The illness is a common killer of orcas and dolphins, the park said.
However marine mammal scientists have argued that keeping orcas in captivity is both physically and psychologically harmful to the animals, and have questioned SeaWorld's claims that orcas' life expectancy in captivity is comparable to the lifespans of orcas in the wild, the BBC reported in 2016.
In an 840-word statement posted on its SeaWorldCares site, the Orlando-based company said Kyara was being treated for an infection.
It was also the end of an era - marking the last captive birth as part of the company's breeding program, which it formally shuttered last March after significant backlash from animal rights activists. Kyara was born in April to mom Takara. In January 2017, a 36-year-old whale Tilikum died in captivity at the park.
The park came under increased scrutiny after the documentary "Blackfish" featured Tilikum, who killed a trainer in 2010..
In a series of afternoon tweets, SeaWorld said: "We are extremely saddened to announce the passing of Kyara, our newest killer whale calf". Oscar-nominated actor James Cromwell was cited for trespassing at a SeaWorld San Diego protest.
People for the Ethical Treatment of Animals released a statement on Monday night, saying: "SeaWorld executives have dollar signs where their eyes should be".