Abstract Stereotyped beliefs about old age and the aging process have proven to strongly promote negative behaviors toward the elderly, with unfavorable influences on their mental and physical health. Therefore, it is important to assess negative aging attitudes with brief but reliable and validated measurement instruments. The psychometric properties of the Portuguese version of the Negative Stereotypes Towards Aging Questionnaire (15 items self-reported) are explored and described in a sample of 302 participants (213 females and 89 males) between the ages of 18 and 59 years old. Confirmatory Factor Analysis shows a one-factor structure, similar to the Spanish version. The internal reliability and mean inter-item correlation showed satisfactory psychometric proprieties. Factorial ANOVA reveals that differences in agism beliefs are mainly related to educational level, with lower negative stereotypes in more educated adults. No differences were found concerning gender. This instrument may be a useful tool to assess negative stereotypes toward old age and the aging process.