Yelkouan shearwaters are 30–38 cm long, with a 76–89 cm wingspan. It has the typically "shearing" flight of the genus, dipping from side to side on stiff wings with few wingbeats, the wingtips almost touching the water. This bird looks like a flying cross, with its wing held at right angles to the body, and it changes from very dark brown to white as the dark upperparts and paler undersides are alternately exposed as it travels low over the sea.
It is silent at sea, but at night the breeding colonies are alive with raucous cackling calls, higher pitched and more drawn out than the Manx shearwater's.
The yelkouan shearwater has a more contrasted appearance than the Balearic shearwater with which its winter range overlaps, since the latter species is brown above and dirty white below. It is very similar to the black-and-white Manx shearwater of the Atlantic, and stray birds out of their usual range are very difficult to identify with certainty.
Also, at least one mixed breeding colony of the yelkouan and the Balearic shearwaters exists on Menorca. A study of these birds recommendeds that a combination of morphological characteristics and DNA sequence data should be required at least for scientific purposes to assign individual birds to either species.