Heart rate variability (HRV) is a measure of autonomic influences on heart rate that has frequently been used as a transsituationally consistent biomarker for cardiovascular health and emotional or cognitive functions. The psychometric properties of HRV however remain unclear. In the present study, we examined the reliability and temporal stability of parasympathetic HRV measures and estimated the portion of variance explained by transsituationally consistent trait variance and by effects of the situation and person–situation interaction with structural equation modeling. The results show good reliability of indices reflecting central parasympathetic control over heart rate and that about 40% of the variance of a single HRV measurement can be explained by effects of the situation and person–situation interaction. An aggregation across at least two measurements may be recommended when using HRV as a transsituationally consistent biomarker or trait.