42 - 45 cm. 103 - 110 cm wingspan. Bill black. Plumage mainly gray and white. Crown, nape and upperparts gray with black patch on eye and dark cap. Blackish M pattern covering upperwings and rump. Tail dark, long; feathers that cover uppertail often mottled white. Underparts white, underwings with dark carpal spot, narrow black training margins. Legs black and flesh color. Males and females are similar.
Juan Fernandez Petrel differs from White-necked (Pterodroma cervicalis) by the absence of white collar on nape, gray crown, black tail, and less black on the main edge of the underwing. Larger than Stejneger´s Petrel (Pterodroma longirostris) and M pattern is different. Hawaiian Petrel (Pterodroma sandwichensis) has darker cap and more extensive black on underwing. May resemble light morph Wedge-tailed Shearwater (Puffinus pacifucis) or Galapagos Petrel (Pterodroma phaeopygia), both of which have thick black underwing-margins.