credulous has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is incredulous.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
disposed to believe on little evidence
“the gimmick would convince none but the most credulous”
1 of the 2 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for credulous.
Synonyms, antonyms and the broader and narrower relations for credulous come from Princeton University’s WordNet. Full sources and licences.