A tonneau cover for covering a cargo box of a pickup truck. The tonneau cover including a cover assembly for covering the cargo box and at least one clamp assembly secured to the cover assembly for securing the tonneau cover to the cargo box. The clamp assembly includes an elongated strap, and a clamp mechanism that is in sliding engagement with the strap between a series of secured positions along the strap. The clamp mechanism includes a clamp that is engageable with the cargo box and a pawl is supported by the clamp and biased into engagement with the strap, or more particularly teeth provided on the strap. A release is also supported by the clamp and by pulling on the release the pawl is disengaged from the strap, allowing the clamp to be slid along the strap.