Khlong Yai was once part of Siam's Patchan Khiri Khet Province. The name was constructed to rhyme with Prachuap Khiri Khan Province on the other side of the Gulf of Thailand at the same latitude. In 1904, most of Patchan Khiri Khet, including Trat, was ceded to France in exchange for the French returning Chantaburi to Siam. Shortly thereafter, France returned Trat and Klong Yai to Siam in exchange for Siem Reap and Battambang, which had been under Thai rule since 1795. A portion of what was Patchan Khiri Khet Province is now in Koh Kong Province of Cambodia.