Background: The COVID-19 pandemic demanded exceptional physical and mental effort from healthcare workers worldwide. Since healthcare workers often refrain from seeking professional psychological support, internet-delivered interventions could serve as a viable alternative option.Objective: We aimed to investigate the effects of a therapist-guided six-week CBT-based internet-delivered stress recovery intervention among medical nurses using a randomized controlled trial design. We also aimed to assess program usability.Methods: 168 nurses working in a healthcare setting (Mage = 42.12, SDage = 11.38; 97 % female) were included in the study. The intervention group included 77 participants, and the waiting list control group had 91 participants. Self-report data were collected online at three timepoints: pre-test, post-test, and three-month follow-up. The primary outcome was stress recovery. Secondary outcomes included measures of perceived stress, anxiety and depression symptoms, psychological well-being, posttraumatic stress and complex posttraumatic stress symp-toms, and moral injury.Results: We found that the stress recovery intervention FOREST improved stress recovery, including psychologi-cal detachment (d = 0.83 [0.52; 1.15]), relaxation (d = 0.93 [0.61, 1.25]), mastery (d = 0.64 [0.33; 0.95]), and control (d = 0.46 [0.15; 0.76]). The effects on psychological detachment, relaxation, and mastery remained stable at the three month follow-up. The intervention was also effective in reducing its users stress (d = -0.49 [-0.80;-0.18]), anxiety symptoms (d = -0.31 [-0.62;-0.01]), depression symptoms (d = -0.49 [-0.80;-0.18]) and increasing psychological well-being (d = 0.53 [0.23; 0.84]) with the effects on perceived stress, depression symptoms, and well-being remaining stable at the three-month follow-up. High user satisfac-tion and good usability of the intervention were also reported.Conclusions: The present study demonstrated that an internet-bas ed intervention for healthcare staff could increase stress recovery skills, promote psychological well-being, and reduce stress, anxiety, and depression symptoms, with most of the effects being stable over three months.Trial registration: NCT04817995 (https://clinicaltrials.gov/ct2/show/NCT04817995). Registration date: March 30, 2021. Date of first recruitment: April 1, 2021.(c) 2022 Elsevier Ltd. All rights reserved.