In an online study using a balanced placebo design, we examined whether decentering, a key component of mindfulness, and expectations, a key component of placebo effects, contribute additively or interactively to reducing anxiety related to the COVID-19 pandemic. 128 adults were randomly assigned to one of four groups: told effective/mindful decentering, told effective/sham decentering, told sham/mindful decentering, told sham/sham decentering.Instructions were given using standardized audio-instructions. Current anxiety was assessed pre- and post-intervention with the short State version of the State-Trait Anxiety Inventory. The data could provide neither evidence for an additive nor an interactive effect of mindful decentering and treatment expectations on anxiety. Compared to sham decentering, mindful decentering reduced anxiety post-intervention. The mindful decentering group mentioned more decentering-related words than the sham decentering group. The findings indicate that a short, standardized, and online mindful decentering intervention can effectively decrease pandemic-related anxiety, independently of one’s expectations. These findings will inform on future research and on methods for optimizing mindfulness effects.