Memristors have attracted tremendous interest in the fields of high-density memory and neuromorphic computing. However, despite the tremendous efforts that have been devoted over recent years, high operating voltage, poor stability, and large device variability remain key limitations for its practical application and can be partially attributed to the un-optimized interfaces between electrodes and the channel material. We demonstrate, for the first time, a van der Waals (vdW) memristor by physically sandwiching pre-fabricated metal electrodes on both sides of the two-dimensional channel material. The atomically flat bottom electrode ensures intimate contact between the channel and electrode (hence low operation voltage), and the vdW integration of the top electrode avoids the damage induced by aggressive fabrication processes (e.g. sputtering, lithography) directly applied to the channel material, improving device stability. Together, we demonstrate memristor arrays with a high integration density of 1010 cm.2, high stability, and the lowest set/reset voltage of 0.12 V/0.04 V, which is a record low value for all 2D-based memristors, as far as we know. Furthermore, detailed characterizations are conducted to confirm that the improved memristor behavior is the result of optimized metal/channel interfaces. Our study not only demonstrates robust and low voltage memristor, but also provides a general electrode integration approach for other memristors, such as oxide based memristors, that have previously been limited by non-ideal contact integration, high operation voltage and poor device stability.