Khlong Saen Saeb is a canal (khlong) in central Thailand, connecting the Chao Phraya River to Prachinburi Province and Chachoengsao. The Saen Saeb starts from Maha Nak Canal (Khlong Maha Nak) around Mahakan Fort in Bangkok. In Chachoengsao Province it ends in the Bang Pakong River. A portion of the canal is used for public transport by an express boat service in Bangkok. The 72 km long canal passes through 21 districts and is connected to more than 100 smaller canals.
Historically the canal was built on the order of King Rama III during a conflict between Siam and Annam over Cambodia in order to establish water transport for soldiers and weapons. Construction began in 1837 and was finished in three years. Klong Saen Saep was once filled with lotus plants. King Mongkut in his fourth reign (1851-1868) built Sa Pathum Palace (Lotus Pond Palace) in Siam District. The name of the palace and present-day Pathum Wan District are derived from it.
The Saen Saeb Canal has an environmental issue being one of Bangkok's major drainage arteries. It is heavily polluted although a two-year clean-up effort was initiated in 2017-2018. A survey concluded that there are 631 potential sources of pollution along the canal, including hospitals, restaurants, markets, hotels, condominiums, department stores, and homes.