Resistive random-access memory (ReRAM) is a promising technology for both memory and for in-memory computing. However, these devices have security vulnerabilities that are yet to be adequately investigated. In this work, we identify one such vulnerability that arises from the write mechanism in ReRAMs. Whenever a cell/row is written, a constant bias is automatically applied to the remaining cells/rows to reduce sneak current. We develop a new attack (referred as WriteHammer) that exploits this process. By repeatedly exposing a subset of cells to this bias, WriteHammer can cause noticeable resistance drift in the victim ReRAM cells. Experimental results indicate that WriteHammer can cause up to 3.5X change in cell resistance by repeatedly writing to the ReRAM cells for a duration of 4 ms.