The formation of grain boundaries (GBs) in graphene films is both fundamentally interesting and practically important for many applications. A GB in graphene is known as a linear defect and is formed during the coalescence of two single crystalline graphene domains. The covalent binding between domains is broadly known as the mechanism of GB formation during graphene chemical vapor deposition (CVD) growth. Here, we demonstrate another GB formation mechanism, where two graphene domains are connected by weak van der Waals interactions between overlapping graphene layers. The formation mechanism of the overlapping GBs (OLGBs) is systematically explored theoretically and the proposed conditions for forming OLGBs are validated by experimental observations. This discovery leads to a deep understanding of the mechanism of graphene CVD growth and reveals potential means for graphene quality control in CVD synthesis.