harmonious has 2 distinct synonyms spread across 1 of its 4 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. Its one recorded opposite is inharmonious.
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.
musically pleasing
exhibiting equivalence or correspondence among constituents of an entity or between different entities
2 of the 4 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for harmonious.
Synonyms, antonyms and the broader and narrower relations for harmonious come from Princeton University’s WordNet. Full sources and licences.