It is a small bird, around 12 cm (4.7 in) long and weighing 12 g (0.42 oz), with a black crown and nape, pale cheeks, brown back and greyish-brown wings and tail.

Its close resemblance to the willow tit can cause identification problems, especially in the United Kingdom where the local subspecies of the two are very similar: they were not recognised as separate species until 1897.

The marsh tit was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Parus palustris.

[4] The genus name, Poecile, is the Ancient Greek name for a now unidentifiable small bird, and is perhaps derived from poikolos, meaning "spotted"; the specific palustris is Latin for "marshy".

[7][8] Ten subspecies are recognised:[9] The nominate race has a black cap and nape with a blue sheen visible at close quarters.

Some of the more common songs include a typical tit-like, ringing, "schip-schip-schip-schip-ship", a more liquid "tu-tu-tu-tu-tu" and sometimes a sweet "tyeu-tyeu-tyeu-tyeu-tyeu".

[12] Marsh and willow tits are difficult to distinguish on appearance alone; the races occurring in the UK (P. p. dresseri and P. m. kleinschmidti respectively) are especially hard to separate.

When caught for ringing, the pale 'cutting edge' of the marsh tit's bill is a reliable criterion;[13] otherwise, the best way to tell apart the two species is by voice.

[17] For example, between the 1970s and 2007, marsh tit numbers declined in the United Kingdom by more than 50% and consequently it is on the Red List of species compiled by the UK's Joint Nature Conservation Committee.

Other studies have shown that reduced diversity of woodland structure and plant species, partly because of the impact of deer browsing, is the cause of the bird's decline.

The study concluded that damage to the shrub layer, caused by overgrazing by deer, for example, may make woodland less suitable for marsh tits.

This species is sedentary, making short post-breeding movements in most of its range, but in northern Europe some move southward in winter.

Marsh tits do not usually excavate their own nest holes, though they may enlarge the hollow, carrying the chips to a distance before dropping them.

A study of marsh tits in Białowieża Forest, Poland, looked at which factors affected birds' choice to re-occupy a nest hole used the previous year.

[23] A study in Bourton Woods, Gloucestershire, England, found that nestboxes were used by marsh tits in successive years in only 20% of cases.

[21] Mostly spiders and insects are eaten in spring and summer, but seeds – including those of the thistle – nuts and berries are taken in autumn and winter.

The hidden seeds are prone to being stolen, by other marsh tits or other species, so birds often fly from one site to another before deciding on a hiding place.