Rück's blue flycatcher (Cyornis ruckii) is a passerine bird in the Old World flycatcher family Muscicapidae. It is known from only four specimens and is endemic to a small area in northeast Sumatra, Indonesia, inhabiting primary lowland forest. Although all specimens share common characteristics, such as a black bill, brown iris, and black feet, two of the collected specimens show some physical discrepancy with the other two. They were initially described as Cyornis vanheysti before being accepted as specimens of C. ruckii. Rück's blue flycatcher has also been compared to other species of Cyornis.

The species is listed as "Critically Endangered" by the International Union for Conservation of Nature as it has not been recorded since 1918. It has been protected by Indonesian law since 1972. It also might have been affected by the 2004 Indian Ocean tsunami.

Rück's blue flycatcher is 17 cm (6.7 in) in length, with a black bill, brown iris and black feet. Differences between the specimens described by Oustalet and Robinson & Kloss exist. In the Oustalet specimens, the male has dark blue plumage, a blue belly, and a shining blue rump. There is also a small amount of grey coloring on disrupted feathers around the legs. The female has rufous-brown plumage, strong rufous lores, orange-rufous breast and a whitish belly. In the Robinson & Kloss specimens, the immature male has brown-spotted buff plumage with a rufous breast and a whitish center on underparts. The adult male shows some discrepancy with the adult male specimen described by Oustalet; the belly and tail coverts are whitish grey, while the flanks are bluish grey. Additionally, the Robinson & Kloss specimens have slightly larger bills. These differences could be caused by individual variation or due to the specimens being of different subspecies.