Royal Thai Navy Submarine Memorial Day, observed in Thailand every September 4, honors the submarine crews that served in the Royal Thai Navy from 1938 to 1951, prior to the decommissioning of Thai submarines. The day commemorates the delivery of the first two Matchanu class submarines to the Royal Siamese Navy on September 4, 1937. These submarines, built by Mitsubishi Heavy Industries, were primarily used for coastal defense and participated in the Franco-Thai War of 1940–1941. Despite their limited operational history and eventual decommissioning after the Manhattan Rebellion in 1951, the Royal Thai Navy continues to celebrate this day to honor those who served in the submarine force and to express hopes for future naval expansion. The observance serves as a reminder of the historical significance of submarines in Thailand's naval history.