A tonneau cover system clamp can include first and second clamp members with first and second clamping surfaces, respectively. The second clamp member can be slidably coupled to an elongate member relative to the first clamp member. A locking mechanism can be movable to engage the elongate member to prevent lateral translation of the second clamping surface relative to the first clamping surface in response to the first and the second clamp members being in the locked position. The locking mechanism can be movable to release the elongate member to allow lateral translation of the second clamping surface relative to the first clamping surface in response to the first and the second clamp members being in the unlocked position. A biasing member can apply a biasing force biasing the first clamp member and the second clamp member toward the proximal end of the elongate member.