A thread clamp, designed for periodically retaining a weft yarn in a loom of the shuttleless type, comprises a housing with a stationary insert having front and rear recesses spanned by respective membranes defining therewith two chambers of variable volume, the membranes being interconnected by a stem passing with clearance through a passage in the insert. One end of the stem, projecting beyond the front membrane, carries a movable jaw normally held against a confronting fixed jaw by a biasing spring disposed in the front compartment. The rear chamber, whose membrane is of larger effective diameter than that of the front chamber, can be expanded by the admission of air under pressure through one or more passages in the insert, the difference in membrane area being sufficient to let the resulting pressure differential in the two chambers overcome the force of the biasing spring to separate the movable jaw from the fixed jaw.