Ratchawithi Road was formerly known as Sang Hi Road (Thai: ถนนซางฮี้ or ถนนซังฮี้). It was originally constructed during the reign of King Chulalongkorn, as one of three new roads constructed at the time Dusit Palace was built, along with Lok Road (Thai: ถนนลก, now Rama V Road) and Duang Tawan Road (Thai: ถนนดวงตะวัน, now Si Ayutthaya Road). The name "Sang Hi" comes from Chinese, and means "exceedingly happy".
The original road stretched only from behind Dusit Palace to the edge of the Chao Phraya. It was later expanded to connect to Ratchaprarop Road, and then extended across the river to Charan Sanitwong Road when the Krung Thon Bridge began construction in 1954. Because it connected the two sections of the former Sang Hi Road on either side of the river, the Krung Thong Bridge has been called by locals Sang Hi Bridge (Thai: สะพานซังฮี้, Saphan Sang Hi) since before it was officially christened. The intersection of Ratchawithi and Samsen roads, the final intersection before crossing the bridge on the east side of the river, is named Sang Hi Intersection.
The name was changed to Ratchawithi Road on February 16, 1918 by King Vajiravudh. The term is derived from the Pali/Sanskrit, rāja + vīthi, and means "royal way" or "king's road".