Microichthys grandis n. sp. from off southwest Ireland, northeastern Atlantic Ocean, is described on the basis of a single specimen and compared with the other species of the genus. The new species is characterized by the following combination of characters: dorsal-fin rays VII–I, vii, 1; total pectoral-fin rays 18; total gill rakers 25; pyloric caeca 3; pored lateral-line scales 41 + 2; vomerine teeth 6, in a single row; denticles on posterior margin of preopercle 3, on lower margin 3; nape not humped; bony rim of orbit raised above dorsal profile; ridge present in interorbital region; lateral line gradually running towards midline of body, crossing last dark lateral bar above its centre; oblique dark lateral bars 20, lower counter-bars 12; vertical black bar on caudal peduncle present, together with another black bar below the end of the second dorsal fin; peritoneum silvery; pelvic fin short than head length; pelvic fins black. With 54.5 mm SL, this is the largest species of the genus. A revised key to the species of Microichthys is presented.