The debate over undoubtably vs undoubtedly trips up a surprising number of writers, and it's easy to see why. Both sound plausible when spoken aloud. Both carry the same intended meaning. But only one of them belongs in standard written English, and getting it wrong can quietly undermine otherwise polished prose.
Which is correct: undoubtably or undoubtedly?
Undoubtedly is the correct word. It's a standard adverb meaning "without doubt," "certainly," or "unquestionably." You'll find it in every major dictionary: Merriam-Webster, Oxford, Cambridge. Undoubtably doesn't appear in any of those dictionaries as a standard entry. It's a misspelling, full stop.
That said, "undoubtably" isn't random gibberish. It follows a pattern that feels grammatically reasonable, which is exactly what makes it stick. Writers form it by analogy with words like "remarkably," "considerably," or "arguably," where an adjective ending in "-able" gains "-ly" to become an adverb. The trouble is that "undoubtable" isn't a widely recognised adjective either. The base form that actually exists in English is "undoubted," and its adverb is "undoubtedly."
Why "undoubtably" feels so natural
English has a large family of "-ably" adverbs: "reliably," "arguably," "invariably," "notably." Speakers absorb this pattern and apply it freely. When someone wants to intensify a statement, the suffix "-ably" bubbles up and attaches to "undoubt-" in a way that feels perfectly formed. The result sounds authoritative. It isn't.
This is the same mechanism behind misspellings explored in pieces like epitomy vs epitome, where the spoken version of a word pulls writers toward a plausible but wrong spelling. The ear is a poor editor. The word you write should match the dictionary, not the sound in your head.
It's worth noting that "undoubtably" has crept into informal digital writing. Some style guides acknowledge its growing use. But as of 2026, no major prescriptive authority accepts it as standard, and using it in professional, academic, or formal writing marks a clear error.
How to use "undoubtedly" correctly
"Undoubtedly" functions as an adverb. It modifies verbs, adjectives, or entire clauses, and it communicates certainty without hedging. Place it at the start of a sentence, before a verb, or before the adjective it modifies.
- She is undoubtedly the strongest candidate in the field.
- Undoubtedly, the decision will face legal challenges.
- The new policy will undoubtedly attract attention.
One caution: "undoubtedly" signals absolute certainty. Don't use it when the situation allows for reasonable doubt. If you mean "probably" or "very likely," those words serve the sentence better. Precision in adverb choice is the mark of careful writing, and it connects to the broader challenge of choosing words that mean exactly what you intend, not just approximately what you want to say.
A quick way to remember the difference
Think of the base adjective. "Undoubted" is the real adjective in standard English, meaning "not open to doubt." Add "-ly" and you get "undoubtedly." There's no standard adjective "undoubtable," so there's no standard adverb "undoubtably." The chain breaks at the first link.
If you catch yourself writing "undoubtably," replace it with "undoubtedly" every time. The meaning you intended stays intact. The error disappears. This is similar to the logic behind practicle vs practical: a misspelling can feel right because it follows a familiar suffix pattern, but the dictionary is the final word.
Other words that cause the same confusion
"Undoubtably" isn't alone in the category of plausible-but-wrong adverbs. "Irregardless" follows the same logic: a negative prefix stacked onto an already-negative word. "Supposably" gets used in place of "supposedly" by analogy with "-able" forms. These errors share a common cause. Spoken English tolerates them. Written English doesn't.
For formal writing, a quick dictionary check before hitting publish catches most of these. The Merriam-Webster entry for "undoubtedly" confirms the correct form in seconds.
The verdict
Use undoubtedly. It's the only standard form. "Undoubtably" follows a logical-sounding pattern but lands outside the accepted vocabulary of formal English. When you need to express certainty without qualification, "undoubtedly" does the job cleanly and correctly.