Jia Jia, whose name means "good", had been gifted to Hong Kong in 1999 along with another panda, to mark the second anniversary of the city's handover from former colonial ruler Britain.
In recent weeks Jia Jia's food consumption had sharply declined from over 10kg (22 pounds) to less than three kg (six pounds) per day and her average weight dropped from 71kg (156 pounds) to around 67kg (147 pounds).
"Over the past few days, she has been spending less time awake and showing no interest in food or fluids. Her condition became worse this morning. Jia Jia was not able to walk about without difficulties and spent the day laying down," Ocean Park said in a statement posted on its website.