An improved modular vehicle accessory clamp for is disclosed. The clamp provides for gross and fine adjustment between the jaws to allow for differences in the width of vehicle roof rack cross bars. The clamp has a first body with a depending jaw and an extension portion and a second body portion also with a depending jaw and a receiving portion that receives the extension of the first body and allows for adjustment of the distance between the jaws. The clamp can be locked to the cross bar. Upper grooves in the clamp allow for easy attachment to vehicle accessories such as rooftop carriers.