(1) Background: The level of stress experienced by staff in the healthcare sector is highly prevalent and well documented. Self-compassion may support the health and wellbeing of individuals and enable them to stay well at work. This study aimed to understand whether a brief, online, self-guided, novel intervention improved the health and wellbeing of healthcare professionals. (2) Methods: In a parallel randomised controlled trial, a volunteer sample of healthcare professionals were assigned to an intervention group (n = 110) or a waitlist control group (n = 80). Measures of self-compassion, mental wellbeing, stress and burnout were collected by an online questionnaire at baseline, post-programme and, for the intervention group, at follow-up. (3) Results: This intervention appeared to be effective in increasing self-compassion and mental health and decreasing stress and burnout. Significant group effects and significant time × group interactions for overall self-compassion [F (2, 183) = 32.72, p < 0.001; effect size η_p² = 0.226], mental wellbeing [F (2, 212) = 17.46, p < 0.001; effect size η_p² = 0.135], perceived stress [F (2, 205) = 5.42, p = 0.006; effect size η_p² = 0.46], personal burnout [F (2, 224) = 7.57, p = 0.001; effect size η_p² = 0.063] and work burnout [F (2, 208) = 7.39, p = 0.001; effect size η_p² = 0.062] were found. (4) Conclusions: This study shows promise that an affordable and scalable intervention can be effective for busy healthcare professionals operating in a significantly challenging environment.