This is the heartwarming moment tourists gathered and sang a happy birthday song to the oldest Hippopotamus in Thailand as she turned 55.

Footage shows the animal named Mae Mali, or Mother Mali, as she enters her large pool enclosure at the Khao Kheow Open Zoo in Chonburi province before the crowd started singing the birthday song for her on Tuesday (September 8).

Zoo director Atthaporn Sriheran with the help of the keepers prepared a 55th birthday cake for her made out of colourful fresh fruits and vegetables.

He said: "The hippo is loved by people and has stayed here for many years. The staff are close to her and she is nice to everyone."

The crowd awed in delight as the beloved hippo began munching on the colourful fruity cake.

Atthaporn added that everyone, including their guests, wished the adorable hippo happiness and good health in the years to come.

Hippos are large, mostly herbivorous, semi-aquatic mammal native to sub-Saharan Africa. They are graceful in water, good swimmers, and can hold their breath underwater for up to five minutes.