An interrogative clause is a clause whose form is typically associated with question-like meanings. For instance, the English sentence "Is Hannah sick?" has interrogative syntax which distinguishes it from its declarative counterpart "Hannah is sick". Also, the additional question mark closing the statement assures that the reader is informed of the interrogative mood. Interrogative clauses may sometimes be embedded within a phrase, for example: "Paul knows who is sick", where the interrogative clause "who is sick" serves as complement of the embedding verb "know".

Languages vary in how they form interrogatives. When a language has a dedicated interrogative inflectional form, it is often referred to as interrogative grammatical mood. Interrogative mood or other interrogative forms may be denoted by the glossing abbreviation INT.