Background: Established psychometric instruments to detect individuals at high-risk of bipolar disorders (BD) are essential to advance preventive approaches. Methods: The Semi-structured Interview for Bipolar At-Risk States (SIBARS)’s psychometric properties were evaluated through: (i) dimensionality (confirmatory factor analysis, CFA); (ii) reliability (internal/inter-rater reliability); and (iii) validity in terms of convergent validity (Hamilton Depression Rating Scale, HAM-D, Mini-International Neuropsychiatric Interview, MINI; Temperament Evaluation of Memphis, Pisa, and San Diego Autoquestionnaire, TEMPS-A; Young Mania Rating Scale, YMRS), divergent validity (Comprehensive Assessment of At-Risk Mental States, CAARMS; Hamilton Anxiety Rating Scale, HAM-A), concurrent criterion validity (Bipolar Prodrome Symptom Interview and Scale–Abbreviated Screen for Patients, BPSS-AS-P). Results: A total of 193 participants were included. The CFA for depression plus mania showed excellent data fit (Root Mean Square Error Approximation = 0.02). Internal (Cronbach's α = 0.90; McDonald's ω = 0.96) and inter-rater (Intra-class Correlation Coefficient = 0.97) reliability were excellent. Convergent validity was confirmed by moderate-to-strong associations between the SIBARS mania scale and the YMRS (β = 0.49, p < 0.001), the SIBARS depression scale and the HAM-D (β = 0.54, p < 0.001), and the SIBARS cyclothymic temperament scale and the TEMPS-A (β = 0.69 p < 0.001). Divergent validity was evidenced by very weak associations between SIBARS and CAARMS' outcomes (χ² = 4.14, p = 0.042, phi = 0.15) or the HAM-A (r = 0.16, p = 0.025). Concurrent validity was indexed by a significant association of SIBARS' researcher-based ratings and BPSS-AS-P participant-based ratings (r = 0.23, p = 0.001). Conclusions: There is convincing psychometric evidence supporting the SIBARS as a reliable and valid instrument for detecting individuals at clinical high-risk of BD. The cross-sectional design of the study, however, did not allow to test the SIBARS' predictive validity.