The main objective of this study was to analyze the relationship between different sociodemographic characteristics, optimism and subjective well-being (SWB) in a sample of Mexican undergraduate students from the University of San Luis Potosi. Additionally, we have tried to identify which specific life domains influenced more on global well-being and how the optimism could explain well-being. Data were collected of 299 participants aged between 17 and 49 years old (M= 19.38; SD= 3.19) through a semi structured interview, considering different relevant sociodemographic characteristics. Dispositional optimism was evaluated using the Life Orientation Test Revised (Scheier, Carver & Bridges, 1994). Subjective well-being included the following measures: Happiness (Subjective Happiness Scale de Lyubomirsky & Lepper, 1999), Life Satisfaction (Satisfaction with Life Scale from Diener, Emmons, Larsen & Griffin, 1985), Positive and Negative Emotions (Positive and Negative Affect Schedule from Watson, Clark & Tellegen, 1988) and Satisfaction with different Life Domains: Job/ Studies, Partner, Health and Leisure (Marrero, Carballeira & Rodriguez, 2007). Through the Test for Kendall’s Tau-b, the relationship between sociodemographic variables, optimism and well-being indicators was analysed. The results showed moderate associations between gender and studies satisfaction, life satisfaction, happiness and optimism. Also, the fact of having a sentimental relationship was related to partner satisfaction; and the job situation was associated with life satisfaction. Pearson correlational analyses between the eight well-being indicators and the optimism showed that all the well-being variables were related each other. Optimism was more associated to all the measures of SWB than the sociodemographic variables; and the relationships of optimism with global measures of well-being were greater than those with the specific life domains. In order to deep in this association between optimism and well-being, the sample was split in three groups in function of the level of optimism. A MANOVA was made to compare the means of well-being, finding that those participants with high and medium optimism, vs. those with low optimism, reported more life satisfaction and positive emotions, lower negative emotions and more satisfaction in all the specific life domains. The MANCOVA, taking the gender as a covariant, showed similar findings. Finally, Multiple Regression Analyses were applied to know the influence of the different specific life domains on the global components of well-being: happiness and life satisfaction. The results showed that all the specific domains were relevant for both criteria variables, but studies satisfaction was the more powerful predictor. In the new multiple regression analyses, the optimsm was included and explained a higher percentage of variance than the measures of satisfaction with life domains, especially in the case of happiness as the criteria. These findings suggested that optimism is a relevant personality trait to improve subjective well-being of individuals.