constitutional has 7 distinct synonyms spread across 2 of its 5 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 unconstitutional.
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.
sanctioned by or consistent with or operating under the law determining the fundamental political principles of a government
“the constitutional right of free speech”
constitutional in the structure of something (especially your physical makeup)
a regular walk taken as a form of exercise
1 of the 5 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 constitutional.
Synonyms, antonyms and the broader and narrower relations for constitutional come from Princeton University’s WordNet. Full sources and licences.