Thesaurus

Synonyms for Sullen include:

atrabilious, bad-tempered, bilious, bristly, brumous, cantankerous, choleric, churlish, cloud-covered, clouded, cloudlike, cloudy, crabbed, crabby, cranky, cross, crotchety, crusty, curmudgeonly, currish, dark, disagreeable, dour, dull, dyspeptic, foggy, fractious, fussy, glowering, glum, gray, grey, grouchy, gruff, grumpy, hazy, heavy, hot-tempered, hotheaded, huffish, ill-humored, ill-humoured, ill-natured, ill-tempered, irascible, irritable, leaden, liverish, lowering, miasmal, miasmic, misanthropic, misanthropical, misogynous, misty, moody, morose, nagging, nebular, nettlesome, ornery, overcast, peckish, peevish, pettish, petulant, prickly, quick-tempered, saturnine, scratchy, shirty, short-tempered, shrewish, smoggy, snappish, snappy, snorty, sour, splenetic, spoiled, spoilt, sulky, sullen, sunless, surly, techy, testy, tetchy, threatening, ugly, unpleasant, vaporous, vapourous, vinegarish, vinegary, waspish

sullen is a ADJ.