Context: Wikipedia has become one of the ten-most visited sites on the Web, and the world's leading source of Web reference information. Its rapid success has attracted over 400 scholarly studies that treat Wikipedia as a major topic or data source. Objectives: This article presents a protocol for conducting a systematic mapping (a broad-based literature review) of research on Wikipedia. It identifies what research has been conducted; what research questions have been asked, and which have been answered and which remain unanswered; and what theories and methodologies have been employed to study Wikipedia. Methods: This protocol follows the rigorous methodology of evidence-based software engineering to conduct a systematic mapping study. Results and conclusions: The study specified by this protocol is currently in progress, and has thus far identified over 400 studies. Sample preliminary results are presented.