Objectives <p>Concerns about the mental health and well-being of K-12 schoolteachers (individuals teaching children aged 5–18 in traditional classroom settings in the USA) have never been more urgent, with stress levels, burnout, and rates of attrition reaching historically high levels. Mindfulness training may be a professional development option for teachers to better meet the demands of their career with skills such as mental flexibility, emotion regulation, resilience, empathy, compassion, and interpersonal skills. This study examined the efficacy of the Mindfulness for Humans (MfH) course, an online, self-directed mindfulness curriculum that is grounded in evidence-based practices, is easily accessible and scalable and relatively brief, and requires little or no work on the part of the school system to implement.</p> Method <p>We utilized a randomized waitlist-controlled trial to examine the feasibility, acceptability, and impact of the MfH course on the well-being and mindfulness skills of 230&#xa0;K-12 teachers of regular classrooms using responses to self-report measures and a satisfaction questionnaire (Group 1, <i>n</i> = 115; Group 2, <i>n</i> = 115).</p> Results <p>Online mindfulness training through the MfH course significantly increased teacher well-being and mindfulness skills compared to a waitlist group (<i>d</i> = 0.28 to 0.58) regardless of initial familiarity with mindfulness practices. Participants reported that the curriculum was engaging and useful.</p> Conclusions <p>Online mindfulness training through the MfH course appears to support the well-being of K-12 schoolteachers. Future research may examine additional outcomes such as teacher stress, resilience, or psychological distress.</p> Preregistration <p>This study is not preregistered.</p>