Adult separation anxiety disorder (ASAD) is classified under anxiety disorders in Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5). However, the reasons for ASAD are uncertain. The relationship between ASAD and childhood experiences is one of the frequently debated issues. The purpose of this study was to develop a model that would examine the mediating roles of insecure attachment dimensions—avoidance and anxiety—in the relationship between adults’ retrospective perceptions of parental acceptance–rejection in childhood and separation anxiety. A total of 1,534 participants completed Parental Acceptance–Rejection Questionnaire, Experiences in Close Relationships–Revised, Adult Separation Anxiety Questionnaire, and personal information form. The results of the structural equation model indicated that insecure attachment dimensions—avoidance and anxiety variables—fully mediated the relationship between perceived parental acceptance–rejection and separation anxiety. These findings shed new light on the relationship between adults’ retrospective perceptions of childhood experiences and adult separation anxiety.