The Brown Thornbill is a small bird, but is one of the medium-sized and more common of the thornbills. It has olive-brown to grey upperparts, with a warm reddish-brown forehead scalloped with paler markings. The rump has a reddish-brown patch, the tail is grey-brown with a black band and a pale tip, and the underparts are off-white, streaked blackish on the chin, throat and chest. The eye is dark red. The sexes are similar and young birds are only slightly different to adults, with a duller eye