Optimism can be defined as the hope that something good is going to happen in the future. It is a relevant construct in the study of happiness, and is associated with a range of variables, including subjective well-being, reduced risk of suicidal ideation, quality of social relationships, and a healthier lifestyle. However, current measures of optimism were criticized regarding their structure and reliability. To address these limitations, Pedrosa et al. (2015) proposed a new scale of dispositional optimism that was originally published in Spanish. In the present research, we aimed to provide further psychometric evidence of the 9-item Optimism Scale in the United Kingdom (N = 325) and Brazil (N = 421). Exploratory and confirmatory factor analyses in both countries were consistent with the original findings, supporting the unifactorial structure. Item Response Theory revealed good discrimination, level of difficulty, and informativeness of the items. Further, we found good reliability estimates of the scale, full factorial invariance across participants' gender and partial invariance across countries, and positive correlations with all Big-5 personality traits. In sum, our findings suggest that the dispositional Optimism Scale is a psychometrically adequate measure that can be used cross-culturally.