Background: Advocacy informed by scientific evidence is necessary to influence policy and planning to address physical inactivity. The mass media is a key arena for this advocacy. This study investigated the perceptions and practices of news media professionals reporting physical activity and sedentariness to inform strategic communication about these issues. Methods: We interviewed media professionals working for major television, radio, newspaper and online news outlets in Australia. The interviews explored understandings of physical activity and sedentariness, attributions of causality, assignment of responsibility, and factors affecting news reporting on these topics. Data were thematically analysed using nVivo. Results: Physical inactivity was recognised as pervasive and important, but tended to be seen as mundane and not newsworthy. Sedentariness was regarded as more novel than physical activity, and more likely to require organisational and environment action. Respondents identified that presenting these issues in visual and engaging ways was an ongoing challenge. Conclusions: Physical activity researchers and advocates need to take account of prevailing news values and media practices to improve engagement with the news media. These include understanding the importance of novelty, narratives, imagery, and practical messages, and how to use these to build support for environmental and policy action.