Thesaurus APIs
Synonym and antonym lookups are the focus. Synonyms API from STANDS4 returns synonyms, antonyms and a short definition in XML or JSON, and WordsAPI adds rhymes and other word relationships. Merriam-Webster and Collins bundle thesaurus data with their dictionaries, Collins in bilingual form. Datamuse is the free, keyless option, finding related words by meaning rather than returning a fixed synonym list.
| API | Pricing | Auth | Format | Free tier | Status |
|---|---|---|---|---|---|
CollinsBilingual dictionary and thesaurus data (definitions, translations, word forms) from HarperCollins. Data & Knowledge › Dictionaries | — | Untested | |||
DatamuseFinds words matching meaning, sound, spelling pattern or related-word constraints for language tools. Data & Knowledge › Dictionaries | Free | None | JSON | 10k+/day | Live |
Merriam-WebsterWord definitions, etymology, audio pronunciation and thesaurus data from Merriam-Webster's reference works. Data & Knowledge › Dictionaries | Freemium | API key | JSON | 1k–10k/day | Live |
Synonyms API (STANDS4)Looks up synonyms, antonyms and a short definition for an English word, in XML or JSON. Data & Knowledge › Dictionaries | Freemium | API key | XML | <100/day | Live |
WordsAPIDefinitions, synonyms, antonyms, rhymes and word relationships for over 150,000 English words. Data & Knowledge › Dictionaries | Freemium | API key | JSON | 1k–10k/day | Live |
CollinsUntested
Bilingual dictionary and thesaurus data (definitions, translations, word forms) from HarperCollins.
·
—
no machine-readable spec
DatamuseFreeLive
Finds words matching meaning, sound, spelling pattern or related-word constraints for language tools.
None · JSON
10k+/day
no machine-readable spec
Merriam-WebsterFreemiumLive
Word definitions, etymology, audio pronunciation and thesaurus data from Merriam-Webster's reference works.
API key · JSON
1k–10k/day
no machine-readable spec
Synonyms API (STANDS4)FreemiumLive
Looks up synonyms, antonyms and a short definition for an English word, in XML or JSON.
API key · XML
<100/day
no machine-readable spec
WordsAPIFreemiumLive
Definitions, synonyms, antonyms, rhymes and word relationships for over 150,000 English words.
API key · JSON
1k–10k/day
no machine-readable spec
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Choosing a Thesaurus API
Is there a free Thesaurus API?
Yes. Datamuse is free to use, and Merriam-Webster, WordsAPI and Synonyms API (STANDS4) have a free tier alongside paid plans.
Which Thesaurus APIs work without an API key?
Datamuse needs no key or sign-up, so you can try it with a plain request.
Can I call a Thesaurus API from front-end JavaScript?
Datamuse allows cross-origin requests and needs no key, so it works straight from the browser. For the others, call them from your server so your key stays private.
Can I use Thesaurus APIs in a commercial project?
Merriam-Webster and Synonyms API (STANDS4) restrict it, for example to non-commercial or attributed use, so read the terms first.
Broader: Dictionaries APIs · Data & Knowledge APIs