A growing body of research demonstrates the value of asking students to write about science for authentic purposes. But which purposes--and, just as importantly, whose purposes--count as authentic? In this theoretical article, we review several conceptions of authentic purpose drawn from science education and literacy education and use these to question the meaning and significance of authenticity in student science writing. Next, we examine the framework of Culturally Relevant Pedagogy (CRP) and ask how it might be used to define authentic purposes for science writing. We offer an additional conception of authentic purpose, one focused on situations where students' purposes for writing about science directly overlap with teachers' purposes for asking students to write. We share an illustrative example from our work as teacher educators that demonstrates how CRP can focus our attention on the types of classrooms and interactions that might create conditions where students becoming increasingly likely to pursue their own purposes through writing. Finally, using CRP as a framework, we offer seven strategies that might help create such situations, and discuss their implications for science educators and science education researchers. We argue that using CRP to operationalize science writing for authentic purposes can push the field forward by suggesting new directions for research and practice.