Refugees from war zones often have missing significant others. A loss without confirmation is described as an ambiguous loss. This physical absence with simultaneous mental persistence can be accompanied by economic, social or legal problems, boundary ambiguity (i.e., uncertainty about who belongs to the family system), and can have a negative impact on mental health. The aim of this study was to identify sociodemographic and loss-related predictors for prolonged grief, anxiety, depression, post-traumatic stress disorder (PTSD) and somatization in treatment-seeking Syrian refugees with post-traumatic stress symptoms in Germany experiencing ambiguous loss. For the present study, data were based on the treatment-seeking baseline sample of the “Sanadak” randomized-controlled trial, analyzing a subsample of 47 Syrian refugees with post-traumatic stress symptoms in Germany experiencing ambiguous loss. Sociodemographic and loss-related questions were applied, along with standardized instruments for symptoms of prolonged grief (ICG), anxiety (GAD-7), depression (PHQ-9), PTSD (PDS-5) and somatization (PHQ-15). Linear regression models were used to predict mental health outcomes. Having lost a close family member and higher boundary ambiguity showed a statistically significant association with higher severity in prolonged grief. The overall model for somatization reached statistical significance, while no predictor independently did. Boundary ambiguity showed a statistically significant positive association with depression, while the overall model showed no statistically significant associations. Boundary ambiguity and missing family members seemed to be important predictors for prolonged grief. These findings support the importance of reunification programs and suggest an inclusion of the topic into psychosocial support structures, e.g., including psychoeducational elements on boundary ambiguity in support groups for traumatized individuals and families experiencing ambiguous loss. Further research is needed for a more detailed understanding of the impact of ambiguous loss on refugee populations.