Breath counting is traditionally used in meditation as one technique for anchoring attention on the present-moment experience of the breath. Developed from this meditative exercise, the Breath Counting Task (BCT) is a 20-minute computer-administered objective measure of mindfulness. In the task, participants are required to count their breaths in cycles of nine, indicating each breath with a button press. Test takers are asked to use different responses to distinguish the first to eighth breath from the ninth terminal breath in each cycle, as well as to indicate a loss of count. The main outcome measure of the task is accuracy, computed as the percentage of correctly counted cycles. Errors can also be scored separately as miscounts (cycles containing fewer or more than 9 breaths) and resets (self-reported loss-of-counts). The BCT has moderate test-retest and split-half reliability. It has good construct validity: Better task performance is associated with greater self-reported mindfulness, less mind wandering, superior sustained attention, better mood, and greater nonattachment. Performance on the test improves selectively with breath-counting training. A MATLAB script for stimulus presentation, data analysis of the BCT, as well as instructions for test administration are provided.