Objectives Affecting approximately one-fifth of college students, test anxiety is a source of academic failure that leads to worse exam performance and academic retention. The present study assessed the efficacy of online mindfulness-based interventions at reducing collegiate test anxiety. Methods Undergraduate college students ( N = 71) were randomly assigned to a six-week mindfulness condition ( n = 24), a three-week mindfulness condition ( n = 21), or a six-week sham mindfulness active control condition ( n = 26). Participants in each condition were assigned five weekly meditations and written reflections. Responses to online surveys captured participants’ reported levels of test anxiety and mindfulness pre- and post-intervention. Results Increases in mindfulness were associated with decreased reports of test anxiety, r (69) =  − .48. Across all conditions, self-reported mindfulness increased, and self-reported test anxiety decreased from pre- to post-test, F (1, 68) = 19.5 and 28.9, p ’s < .001, η 2 = .06 and .05 respectively. Conclusions Changes in the primary variables did not differ by condition, offering no empirical support for greater efficacy of a six-week as compared to a three-week intervention. The sham mindfulness control group also reported increases in mindfulness and decreases in test anxiety, raising questions about what intervention benefits are specific to mindfulness trainings. These findings suggest that online interventions are effective at reducing test anxiety in college students, though more research will inform how intervention length and content lead to changes in test anxiety and mindfulness. Supplementary Information The online version contains supplementary material available at 10.1007/s12671-022-02002-6.