belong has 2 distinct synonyms spread across 2 of its 6 senses. 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
be owned by; be in the possession of
“This book belongs to me”
be suitable or acceptable
“This student somehow doesn't belong”
be in the right place or situation
“Where do these books belong?”
Synonyms, antonyms and the broader and narrower relations for belong come from Princeton University’s WordNet. Full sources and licences.
be rightly classified in a class or category
“The whales belong among the mammals”
be a member, adherent, inhabitant, etc. (of a group, organization, or place)
“They belong to the same political party”