In line with the perspective provided by the intentional paradigm which claims that the measures of subjective well-being (SWB), whether cognitive or affective, should refer to life itself, the Ontological Well-Being Scale (OWBS) has been operationalized. The research reported herein was aimed at developing and validating this psychometric tool in the assessment of individuals' affective evaluations of their life within a three-time perspective. Five studies were conducted to confirm the factor structure of the OWBS and to assess its construct validity. Four factors were derived and validated, which were shown to relate to mental health indicators and personality in expected ways. It was found that this new construct, in contrast with the current measures, did not tap into the personality factors of extraversion and neuroticism. Incremental validity results showed that the OWBS explained additional variance in mental health indicators already captured by the current measures of SWB.