Domestic violence is a worldwide crime recognized as a severe violation of Human Rights, which includes Intimate Partner Violence (IPV). The studies remark that the asymmetries in the social relations between men and women result in domination dynamics. Thus, this study analyzed the relationship between gender and IPV beliefs in the general population, university students, and healthcare/safety/justice professionals by comparing IPV legitimization between men and women and with age. The sample was composed by 3413 Portuguese participants, 1551 men (45.4%) and 1826 women (54.6%), aged 18 to 100 (M = 37.97; SD = 18.09), 1936 participants from the general population (56.7%), 866 university students [e.g., healthcare students] (25.4%) and 611 healthcare/safety/justice professionals [e.g., doctors, psychologists, police officers, lawyers] (17.9%). The sample filled out the Scale of Beliefs about Marital Violence (ECVC), a self-report scale on beliefs about IPV. Results confirmed our hypothesis that men have significantly higher levels of IPV legitimization than women. In accordance with our second hypothesis, significant positive correlations were found between age and IPV beliefs. As age increases, older people tend to be more tolerant of IPV, and young people tend to be less endorsing such IPV beliefs. Finally, we found the hypothesis that university students and healthcare/safety/justice professionals have lower levels of beliefs compared with other participants in the general population. Findings show that we need to work hard with the social evolution in men’s and women’s beliefs on IPV, reinforcing the importance of targeting IPV prevention by gender and age in the general population but also in students and professionals.