Mu Ko Phetra is a marine national park in the Andaman Sea off Thailand, covering mostly intact coastal line, open water, and about 30 islands of the southern part of Trang Province and the northern part of Satun Province. Established on 31 December 1984, it is the 49th national park and 14th marine national park of Thailand.
The majority of the park, about 94.74% or 468.38 km² out of 494.38 km² is open water. The two largest islands of the park are Ko Phetra (Thai: เกาะเภตรา) and Ko Khao Yai (Thai: เกาะเขาใหญ่). Several islands are marine turtle egg laying sites. Rich coral reefs are present around islands. Many of the islands consist of steep limestone rocks and little beaches. They are used as temporary refuges by fishermen during fishing season.
In the caves of the cliffs high above the ground on some of the islands are where swiftlets build their nest and breed. Several islands were placed under concession for decades to collect bird nests. They are highly valuable and closely guarded with firearms. Traveling to those islands should be done with local tour companies as their boats are recognized and ignored by guards.
The name Phetra is a Thai corruption of its original Malay name, Pulau Petra (the island of Petra).
The continued existence of the park is threatened by the proposed Pak Bara deepwater seaport.