The American Redstart is a smallish warbler. It measures 11 to 14 cm (4.3 to 5.5 in) in total length, has a wingspan of 16 to 23 cm (6.3 to 9.1 in) and weighs 6.7 to 12 cm (2.6 to 4.7 in). Among standard measurements, the wing chord is 5.5 to 6.9 cm (2.2 to 2.7 in), the tail is 4.9 to 5.8 cm (1.9 to 2.3 in), the bill is 0.7 to 0.9 cm (0.28 to 0.35 in) and the tarsus is 1.5 to 1.9 cm (0.59 to 0.75 in). The breeding males are unmistakable, jet black above apart from large orange-red patches on their wings and tails. Their breast sides are also orange, with the rest of their underparts colored white. In their other plumages, American Redstarts display green in their upperparts, along with black central tails and grey heads. The orange patches of the breeding males are replaced by yellow in the plumages of the females and young birds. Orange and yellow coloration is due to the presence of carotenoids; males possess the red carotenoid Canthaxanthin and the yellow carotenoids Canary Xanthophyll A and B, all of which mix together to produce an orange color, while the females possess only the yellow carotenoids. Recent research indicates that an age and sex effect on observed color attributes of hue, brightness, and saturation exists in American Redstarts, with the exception for saturation, which only showed an age effect. Their song is a series of musical see notes. Their call is a soft chip.

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