The Monumental Cork Oak, also known has The Whistler Tree, is a 236 year old cork oak from Águas de Moura, Palmela, Portugal. It was voted European Tree of the Year in 2018, it has been classified as "Tree of Public Interest" since 1988 and is registered in the Guinness Book of Records as "the largest cork oak in the world". It is around 16.2 m (53 ft) tall with a circumference at breast height of 4.15 m (14 ft).

It was planted in 1783–1784, at that time the Kingdom of Portugal and the Algarves reigned by Queen Maria I and, since 1820, it has been harvested over 20 times.

1991 was a particularly prolific year, as it yielded over 1,200 kg (2,646 lb) of cork, producing more than 100.000 cork stoppers, more than what an average cork tree would produce in its lifetime.

The name 'Whistler Tree' comes from the whistling sound of the birds that land on its branches. The tree was almost taken down with other 411 cork oaks in 2000, as an illegal urban expansion took place. By 2001 the law was reformed to better protect the oaks.