The Rueck's Blue-flycatcher, Cyornis ruckii, also known as Rueck's Niltava, is a medium-sized, up to 17cm old world flycatcher with a black bill, brown iris and black feet. The male has dark blue plumage, white belly, and shining blue rump. The female has rufous brown plumage, rusty breast and whitish belly. The young has brown spotted buff plumage with rufous breast and whitish center on underparts.

The Rueck's Blue-flycatcher is endemic to the island of Sumatra, Indonesia. It is known only from four specimens. Two specimens, an immature and adult male were last recorded and collected around 1917-1918 in secondary lowland forests in Medan area of North Sumatra province by the Dutch collector, August van Heijst. The other two skins are of doubtful origin.

The name commemorates M. Rück, who collected the previous two skin specimens in Malacca.

Due to ongoing habitat loss, small population size and limited range, the Rueck's Blue-flycatcher is evaluated as Critically Endangered on the IUCN Red List of Threatened Species. It is listed on Appendix II of CITES and is protected under Indonesian law since 1972.