The ruby-crowned kinglet (Corthylio calendula) is a very small passerine bird found throughout North America.

The ruby-crowned kinglet is a very small bird, being 9 to 11 cm (3.5 to 4.3 in) long, having a wingspan of 16 to 18 cm (6.3 to 7.1 in), and weighing 5 to 10 g (0.2 to 0.4 oz).[15] It has gray-green upperparts and olive-buff underparts. It has two white wingbars and a broken white eye ring.