Quantum gravity has been a central challenge in theoretical physics. Recently, with the fast development of quantum information science, there is a growing interest in exploring a different parameter space of quantum gravity — superposition of massive objects, and their entanglement and decoherence caused by gravity. We explore the gravitational superposition, entanglement and decoherence of inflationary cosmological perturbations. A boundary term is identified which causes faster decoherence than the previously understood rate from bulk interactions. This gravitational decoherence can be considered together with the recent studied quantum noise of cosmological perturbations, and shed light for the observational search for quantum gravity effects.