This study was conducted to develop an inventory that measures students' motivation to engage in critical reasoning in online discussions. Inventory items were developed based on theoretical frameworks and then tested on 168 participants. Using exploratory factor analysis, test-retest reliability, and internal consistency, twenty-two items were selected for the Motivation for Critical Reasoning in Online Discussions Inventory. Six motivational constructs were supported: Interest and Enjoyment, Normative Goals, Outcome Goals, Implicit Theories, Self-Efficacy Perception, and Finishing Requirements. The inventory can be used by other researchers or online instructors to collect students' motivational data to investigate strategies to promote student motivation in online discussions. (Contains 1 note and 3 tables.)