The fast-acting clamp for a musical instrument is selectively attached to an extending support and has a bracket, a connector, a threaded rod and a positioning assembly. The bracket is U-shaped. The connector is attached to the bracket. The threaded rod extends through the bracket. The positioning assembly has a base, a primary retainer, a secondary retainer and two springs. The base is attached to the bracket. The primary retainer is mounted pivotally on the base and has a tab, a semicircular notch and two edges. The secondary retainer is mounted pivotally on the base and has a semicircular notch and two edges. The springs connect to the base and the retainers to press the retainers inward. The tab is pressed to release the threaded rod so the fast-acting clamp can be quickly attached to or released from an extending support.