Mindfulness apps are becoming popular treatments for chronic pain and mental health, despite mixed evidence supporting their efficacy. Furthermore, it is unclear whether improvements in pain are due to mindfulness-specific effects or placebo effects because no trials have compared mindfulness against a sham control. The objective of this study was to compare mindfulness against 2 sham conditions with differing proximity to mindfulness to characterize the relative contributions of mindfulness-specific and nonspecific processes on chronic pain. We assessed changes in pain intensity and unpleasantness and mindfulness-specific and nonspecific pain-related processes in 169 adults with chronic or recurrent pain randomized to receive a single 20-minute online session of mindfulness, specific sham mindfulness, general sham mindfulness, or audiobook control. Mindfulness was not superior to shams for reducing pain intensity or unpleasantness, and no differential engagement of theorized mindfulness-specific processes was observed. However, mindfulness and both shams reduced pain unpleasantness relative to audiobook control, with expectancy most strongly associated with this effect. Sham specificity had no influence on expectancy or credibility ratings, pain catastrophizing, or pain effects. These findings suggest that improvements in chronic pain unpleasantness following a single session of online-delivered mindfulness meditation may be driven by placebo effects. Nonspecific treatment effects including placebo expectancy and pain catastrophizing may drive immediate pain attenuation rather than theorized mindfulness-specific processes themselves. Further research is needed to understand whether mindfulness-specific effects emerge after longer durations of online training.