This leaf warbler is strongly migratory, wintering mainly in south China and adjacent areas of southeast Asia, although in recent decades increasing numbers have been found in Europe in autumn.

It has greenish upperparts and white underparts, a lemon-yellow rump, and yellow double wingbars, supercilia and central crown stripe.

[4] The genus Phylloscopus, first described by German zoologist Friedrich Boie in 1826, comprises about 50 species of small insectivorous Old World woodland warblers that are either greenish or brown above and yellowish, white or buff below.

[9][10] Although field naturalists, such as Gilbert White in the 18th century and William Edwin Brooks in the 19th, had noted the importance of calls in separating often very similar-looking leaf warblers,[11][12] for many years their views were not always accepted by the ornithological establishment.

In the case of the former subspecies of Pallas's leaf warbler, even though they differ only slightly in plumage, the southern forms are very distinctive vocally.

Their songs and calls differ from those of the nominate race, and DNA analysis from 2006 has confirmed these forms to be sufficiently distinct that they are now treated as separate species, leaving Pallas's leaf warbler as a monotypic taxon.

[18] In Asia, Pallas's leaf warbler can be distinguished from its former subspecies by its yellower head stripes, wingbars and throat as well as its different vocalisations.

[17] The sexes of Pallas's leaf warbler have similar plumage, but non-breeding birds are somewhat brighter green above and have broad, bright fringes to their flight feathers.

[22][23] Pallas's leaf warbler breeds in Siberia from the Altai Mountains east to the Sea of Okhotsk, northern Mongolia, northeast China and possibly North Korea.

In winter, it uses a wider range of habitats, including broadleaf forest and scrub as well as conifers, and can be found in river valleys down to sea level.

[25] The German ornithologist Heinrich Gätke, who moved to the then-British island of Heligoland in 1837 and stayed there for some fifty years, subsequently showed that several Asiatic species, including an occasional Pallas's leaf warbler, were regularly found there in autumn.

[32] Most Pallas's leaf warblers found in Europe are first-year birds,[32] and several reasons for the large increase in numbers in autumn have been proposed.

In the past, these warblers were widely considered to be vagrants or reverse migrants, but were more recently thought to be undertaking a regular migration, taking advantage of the mild oceanic climate on the western fringes of Europe for overwintering.

[33] A flaw in that theory is that many birds should winter in Spain, particularly in the northwest, but Pallas's leaf warbler is rare in that country and tends to occur in the east.

Spanish ornithologist Eduardo de Juana has therefore proposed that once the warblers reach northwest Europe, they then reorientate to a south easterly direction.

It is a round or elliptical cup made from twigs, leaves and other vegetation and lined with finer material including feathers, hair or fine grasses.

[35] Like its relatives, Pallas's leaf warbler is insectivorous, feeding on the adults, larvae and pupa of small insects including flies, moths and aphids; spiders are also taken.