A nonmanual feature, also sometimes called nonmanual signal or sign language expression, is any feature of a sign language aside from those produced using the hands, including body postures and facial expressions. Nonmanual features are necessary components of the grammars of sign languages, in the same way that manual features are. Nonmanual features serve many functions, including negation, grammatical mood, and some functions often served by intonation in spoken languages. Expression is one of five components of a sign, along with handshape (dez), orientation (ori), location (tab), and movement (sig). A major component of expression is mouthing. However, not all signs have an inherent expression. ^{[citation needed]}

Nonmanual features in signed languages do not function the same way that general body language and facial expressions do in spoken ones. In spoken languages, they can give extra information but are not necessary for the receiver to understand the meaning of the utterance (for example, an autistic person may not use any facial expressions but still get their meaning across clearly, and people with visual impairments may understand spoken utterances without the need for visual aides). Conversely, nonmanual features are needed to understand the full meaning of many signs, and they can drastically change the meaning of individual signs. For example, in ASL the signs HERE and NOT HERE have the same manual sign, and are distinguished only by nonmanual features.

Nonmanual features also do not function the same way as gestures (which exist in both spoken and signed languages), as nonmanual features are grammaticised. For this reason, nonmanual features need to be included in signwriting systems.

In sign languages, the hands do the majority of the work, forming phonemes and giving denotational meaning. Extra meaning, however, is created through the use of nonmanual features. Despite the literal meaning of manual, not all signs that use other body parts are nonmanual features of the language, and it generally refers to information expressed in the upper half of the body such as the head, eyebrows, eyes, cheeks, and mouth in various postures or movements.

Nonmanual features have two main aspects—place and setting. These are the nonmanual equivalents to HOLM (handshape, orientation, location, and movement) in manual sign components. Place refers to the part of the body used, while setting refers to the state it is in. For example, the Auslan sign for WHY has nonmanual features necessary to distinguish it from the sign BECAUSE. One of these nonmanual features can be described as having the place of [eyebrows] and the setting of [furrowed].

Although it is done using the face, mouthing is not always considered a nonmanual feature, as it is not a natural feature of signed languages, being taken from the local spoken language(s). Because of this, there is debate as to whether mouthing is a sign language feature or a form of codeswitching.

Many lexical signs use nonmanual features in addition to the manual articulation. For instance, facial expressions may accompany verbs of emotion, as in the sign for angry in Czech Sign Language.

Nonmanual elements can be lexically contrastive. An example is the ASL sign for NOT YET, which requires that the tongue touch the lower lip and that the head rotate from side to side, in addition to the manual part of the sign. Without these features the sign would be interpreted as LATE. Mouthings can also be contrastive, as in the manually identical signs for DOCTOR and BATTERY in Sign Language of the Netherlands.