Background There are numerous barriers that limit access to evidence‐based treatment for posttraumatic stress disorder (PTSD). Internet‐based guided self‐help is a treatment option that may help widen access to effective intervention, but the approach has not been sufficiently explored for the treatment of PTSD. Methods Forty two adults with DSM‐5 PTSD of mild to moderate severity were randomly allocated to internet‐based self‐help with up to 3 h of therapist assistance, or to a delayed treatment control group. The internet‐based program included eight modules that focused on psychoeducation, grounding, relaxation, behavioural activation, real‐life and imaginal exposure, cognitive therapy, and relapse prevention. The primary outcome measure was reduction in clinician‐rated traumatic stress symptoms using the clinician administered PTSD scale for DSM‐V (CAPS‐5). Secondary outcomes were self‐reported PTSD symptoms, depression, anxiety, alcohol use, perceived social support, and functional impairment. Results Posttreatment, the internet‐based guided self‐help group had significantly lower clinician assessed PTSD symptoms than the delayed treatment control group (between‐group effect size Cohen's d = 1.86). The difference was maintained at 1‐month follow‐up and dissipated once both groups had received treatment. Similar patterns of difference between the two groups were found for depression, anxiety, and functional impairment. The average contact with treating clinicians was 2½ h. Conclusions Internet‐based trauma‐focused guided self‐help for PTSD is a promising treatment option that requires far less therapist time than current first line face‐to‐face psychological therapy.