Wat Ratchabophit Sathitmahasimaram Ratchaworawihan was built in 1869 during the reign of King Chulalongkorn (Rama V). It is not far from Wat Pho and can be reached by walking from Tha Tian Pier on the river, or a shorter walk from the Sam Yot MRT stop. The wat displays a mixture of architectural styles, including Thai, Khmer, Sri Lankan and Italian. The ornate hand-painted, glazed tiles surrounding the entire complex are an absolute marvel! The ubosot (ordination hall) was closed off when I visited, so these images are only from the exterior.
The wat is one of the six First Class Royal temples in Bangkok (10 according to some sources).