A split infinitive or cleft infinitive is an English-language grammatical construction in which a word or phrase, usually an adverb or other adverbial, comes between the marker to and the bare infinitive (uninflected) form of a verb. A famous split infinitive occurs in the opening sequence of the Star Trek television series: to boldly go where no man has gone before. Here, the adverb "boldly" splits the full infinitive "to go." More rarely, the term compound split infinitive is used to describe situations in which the infinitive is split by more than one word: The population is expected to more than double in the next ten years.

As the split infinitive became more popular in the 19th century, some grammatical authorities sought to introduce a prescriptive rule against it. The construction is still the subject of disagreement among native English speakers as to whether it is grammatically correct or good style. Henry Fowler wrote in 1926, "No other grammatical issue has so divided English speakers since the split infinitive was declared to be a solecism in the 19c: raise the subject of English usage in any conversation today and it is sure to be mentioned." However, most experts on language now agree that the split infinitive is sometimes appropriate.

Those who use it consciously may see it as a form of hyperbaton (as discussed below), and it has been employed to good effect by some major poets. In Old English, most infinitives were single words ending in -an (compare modern German and Dutch -en), but about one quarter were "to" followed by a verbal noun in the dative case, which ended in -anne or -enne. In Middle English, the bare infinitive and the infinitive after "to" took on the same uninflected form.

The "to" infinitive was not split in Old or Early Middle English. The first known example in English, in which a pronoun rather than an adverb splits the infinitive, is in Layamon's Brut (early 13th century):

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