The willow tit was described in 1827 by the Swiss naturalist Thomas Conrad von Baldenstein under the trinomial name Parus cinereus montanus.

[3] The willow tit is now placed in the genus Poecile that was erected by the German naturalist Johann Jakob Kaup in 1829.

[4] The genus name, Poecile, is the Ancient Greek name for a now unidentifiable small bird, and the specific montanus is Latin for "of the mountains".

It was promoted to species status based on a 2002 phylogenetic analysis that compared DNA sequences from the mitochondrial cytochrome-b gene.

[12] In the east of its range it is much paler than marsh tit, but as one goes west the various races become increasingly similar, so much so that it was not recognised as a breeding bird in Great Britain until the end of the 19th century, despite being widespread.

The willow tit is distinguished from the marsh tit by a sooty brown instead of a glossy blue black cap; the general colour is otherwise similar, though the under parts are more buff and the flanks distinctly more rufous; the pale buff edgings to the secondaries form a light patch on the closed wing.

[13] The willow tit excavates its own nesting hole, even piercing hard bark; this is usually in a rotten stump or in a tree, more or less decayed.

[19] The willow tit has an extremely large range with an estimated population of between 175 and 253 million mature individuals.

This large population appears to be slowly decreasing but the decline is not sufficiently rapid to approach the threshold of vulnerability.