Mindfulness-based interventions have proven effective for the transdiagnostic treatment of heterogeneous anxiety disorders. So far, no study has investigated the potential of mindfulness-based treatments when delivered remotely via the Internet. The current trial aims at evaluating the efficacy of a stand-alone, unguided, Internet-based mindfulness treatment program for anxiety. Ninety-one participants diagnosed with social anxiety disorder, generalized anxiety disorder, panic disorder, or anxiety disorder not otherwise specified were randomly assigned to a mindfulness treatment group (MTG) or to an online discussion forum control group (CG). Mindfulness treatment consisted of 96 audio files with instructions for various mindfulness meditation exercises. Primary and secondary outcome measures were assessed at pre-, posttreatment, and at 6-months follow-up. Participants of the MTG showed a larger decrease of symptoms of anxiety, depression, and insomnia from pre- to postassessment than participants of the CG (Cohen's d(between)=0.36-0.99). Within effect sizes were large in the MTG (d=0.82-1.58) and small to moderate in the CG (d=0.45-0.76). In contrast to participants of the CG, participants of the MTG also achieved a moderate improvement in their quality of life. The study provided encouraging results for an Internet-based mindfulness protocol in the treatment of primary anxiety disorders. Future replications of these results will show whether Web-based mindfulness meditation can constitute a valid alternative to existing, evidence-based cognitive-behavioural Internet treatments. The trial was registered at ClinicalTrials.gov (NCT01577290).