Objective: Existential anxiety (EA) is a construct that refers to fears that are provoked by core threats of human existence, such as death, meaninglessness, and fundamental loneliness. The objective of this study was to develop an EA measure that can be used in research and clinical practice. Method: The Existential Concerns Questionnaire (ECQ) was completed by a nonclinical sample of 389 adults, together with questionnaires measuring death anxiety, intolerance of uncertainty, neuroticism, distress, meaning, and life events. Adaptations were made based on item analysis and factor analysis. A total of 99 adults who had an anxiety and/or depressive disorder completed the final version. Results: The ECQ was demonstrated to be essentially unidimensional and showed good reliability and stability. Correlations with other measures were within the expected range of strength, except for a weak association with life events. Conclusion: Initial results regarding the psychometric properties of the ECQ are promising.