Introduction: SIN-E-STRES is an adjunct Internet-based intervention for patients with posttraumatic stress disorder (PTSD). Patients interact with the program via a weekly monitoring email and by visiting a website that provides access to psychoeducational information and distance counseling. Aim: to evaluate the acceptability and feasibility of SIN-E-STRES. Methods: forty-five adult PTSD patients were registered to use SIN-E-STRES for three months. This study explores feasibility in terms of patients’ interest in joining the program, reported connectivity and use of the website components, and adherence to monitoring. Acceptability is evaluated through the satisfaction expressed by the participants, their opinions regarding the usefulness of the program, and their evaluation of its positive and negative aspects. Results: most of the patients invited to the program were interested in using SIN-E-STRES, and 73% of the registered patients completed at least one of the monitoring assessments and most of them interacted with the website. The overall monitoring response rate was 35%. Most patients stated that the feedback messages helped them remember themes covered in therapy, they also valued the psychoeducational resources on the website. Conclusions: The program may be especially useful for patients who are frequent Internet and email users. The intervention can be enhanced by improving the monitoring and feedback components as well as by adding complementary psychoeducational information. More research about adherence and dropout of PTSD interventions is necessary. The alignment and coherence of SIN-E-STRES with the main treatment contributes to reinforcing treatment indications and consequently may promote treatment adherence and recovery.