Participants performed a categorization training task, where each trial presented an example scenario in which an individual makes a claim based on an observation, and participants marked which fallacy or bias, if any, the individual in the scenario was committing. In two studies, we measure the effect of this training task on critical thinking, measured using an open-ended critical thinking assessment, both pre- and post-training. In Study 1, we pilot these materials in an online college course across a full academic semester and observe credible improvements in critical thinking performance. In Study 2, we conduct a pre-registered randomized controlled experiment using online research participants and observe credible improvements in critical thinking relative to no training, and relative to comparable learning activities focused on conventional curricular content. We infer that the categorization training task facilitated inductive learning of patterns of biased and flawed reasoning, which improved participants’ ability to detect and identify such patterns in the delayed open-ended critical thinking assessment. Such categorization training shows promise as an effective and practical method for improving learners’ resistance to online disinformation.