This document defines PRoPHET, a Probabilistic Routing Protocol using History of Encounters and Transitivity. PRoPHET is a routing protocol for intermittently connected networks, where there is no guarantee that a fully connected path between source and destination exists at any time, rendering traditional routing protocols unable to deliver messages between hosts. These networks are examples of networks where the Delay-Tolerant Network architecture[1] is applicable. The document presents an architectural overview followed by the protocol specification.