In linguistics, an adverbial phrase ("AdvP") is a multi-word expression operating adverbially: its syntactic function is to modify other expressions, including verbs, adjectives, adverbs, adverbials, and sentences. Some grammars use the label adverb phrase to denote an adverbial phrase composed entirely of adverbs versus an adverbial phrase, which might not contain an adverb.

Adverbial phrases can be divided into two types: complementary phrases and modifying phrases. For example, very well is a complementary adverbial phrase that complements "sang" in the sentence "She sang very well". More specifically, the adverbial phrase very well contains two adverbs, very and well: while well qualifies the verb to convey information about the manner of singing. By contrast, almost always is a modifying adverbial phrase that modifies "skip" in the sentence "I almost always skip breakfast."

The following examples illustrate some of the most common types of adverbial phrases. All adverbial phrases appear in bold; when relevant, the head of each adverbial phrase appears in square brackets.

The heads of each of the following adverbial phrases are degree adverbials (written "Deg" in syntactic trees). Degree adverbials modify adjacent adverbs (that is, an adverb that is lower in the syntactic tree than the degree adverbial). Degree adverbials are commonly used in English to convey the intensity, degree, or focusing of an adjacent adverb. In most cases, a degree adverbial is used to modify an adverb in an adverbial phrase: for example, in (1) the degree adverbial very modifies the adverb quickly; in (2) the degree adverbial extremely modifies the adverb hard; in (3) the degree adverbial really modifies the adverb well; and in (4), the degree adverbial so modifies the adverb soon.

Modifying adverbial phrases combine with a sentence, and the removal of the adverbial phrase yields a well-formed sentence. For example, in (5) the modifying adverbial phrase in an hour can be removed, and the sentence remains well-formed (e.g., I'll go to bed); in (6) the modifying AdvP three hours later can be omitted, and the sentence remains well-formed (e.g., We arrived); and in (7), the modifying AdvP before long can be omitted, and the sentence remains well-formed (e.g. The situation had been resolved). Just as adjective phrases function attributively to give additional information about an adjacent noun, the modifying adverbial phrases illustrated in (5) to (7) function as secondary predicates that give additional temporal information about the sentence.

Complement adverbial phrases are much less common than their modifying counterparts. Adverbial phrases functioning as complements usually arise when an adverb licenses a complement as a selectional requirement. Nearly all of these complements license an adjoining prepositional phrase. Below are a few examples of complement adverbial phrases.

The following sentences illustrate the difference between adverbs, adverbial phrases, and adverbial clauses.

In the first example, "soon" is an adverb (as distinct from a noun or a verb), which is a type of adverbial. In the second sentence, the modifying phrase "in an hour" has the same syntactic function (that is, to act adverbially and modify the base of the sentence "I'll go to bed"), though it does not contain an adverb. This modifying phrase includes a preposition and a determiner phrase, and functions as an adverbial, thus making it an adverbial phrase. In the third example, we see a whole clause functioning as an adverbial; it is termed an adverbial clause.