After traumatic experiences, it is common to think about alternative scenarios or outcomes of the event. This is often referred to as counterfactual thoughts (CFT), and CFT after trauma have been associated with posttraumatic stress reactions (PTSR). In this study, we aimed to: (1) investigate the relationship between the vividness and frequency of CFT and PTSR, and to (2) examine the associations between exposure, peri‐traumatic reactions, physical injury and the loss of someone close, and the subsequent vividness and frequency of CFT. The participants (N = 289) were survivors of a terror attack in Norway. More vivid and frequent CFT were significantly and independently related to more PTSR. Exposure during trauma was significantly associated with more vivid CFT in the unadjusted regression model. No further significant relationships were found between exposure, peri‐traumatic reactions, physical injury and the loss of someone close, and the vividness and frequency of CFT. As CFT are common after trauma and potentially cause distress, clinicians should identify and validate CFT, and provide coping assistance.