The Moine Thrust Belt of the Northwest Highlands of Scotland represents the front of the Caledonian Orogeny. The exact age of this thrust belt is the source of continuing debate, and has not been fully resolved either by dating of igneous rocks within the thrust belt or by direct dating of mylonites. The Canisp Porphyry sills are porphyritic quartz–microsyenite intrusions, found in the Foreland to the Moine Thrust Belt in the Assynt region. The sills occur close to the lowest thrust, but are never seen within the thrust belt. They are thus considered to predate movement on the lowest thrusts. This paper presents a U–Pb (TIMS) zircon date of 437 ± 4.8 Ma for the Canisp Porphyry, and thus movement on the lower thrusts of the Moine Thrust Belt is constrained as having occurred after that date.