Background The DSM-5 Alternative Model for Personality Disorders introduced a dimensional perspective on personality disorders. The model assesses functioning in four domains: Identity, Self-Direction, Empathy, and Intimacy. This study evaluates the psychometric properties of the Semi-Structured Interview for Personality Functioning DSM-5 (STiP-5.1) in Estonian. Method The sample consists of 131 participants: 58 from the general population and 73 from a mixed clinical sample that is further divided into a mood and anxiety disorder sample and personality disorder sample. All participants completed the STiP-5.1 interview and the Level of Personality Functioning Scale–Brief Form (LPFS-BF 2.0). Results The Estonian STiP-5.1 interview has good internal consistency (McDonald's ω between .94–.98) and high convergent validity (correlations with LPFS-BF 2.0 above .7). Interview scores successfully differentiated the general population from the mixed clinical sample (Cohen’s d = 2.68), as well as patients with personality disorder from those without (Cohen’s d = 1.76). The LPFS-BF 2.0 total score differentiates the general population sample from the mixed clinical sample (Cohen’s d = 1.99) but not the personality disorder sample from other clinical sample participants. Conclusions The properties of the Estonian STiP-5.1 replicate those of other languages, and empirically support a unified personality functioning dimension that can be meaningfully thought of as reflecting impairments in self and interpersonal functioning. Findings of this study will be discussed in the light of the ongoing debate on the dimensionality of personality pathology and the use of self-report versus interview measures for assessing personality pathology.