Conspiracy theories contribute to vaccine hesitancy and pose a serious challenge to vaccination campaigns. The Vaccine Conspiracy Beliefs Scale (VCBS) is a short 7-item scale to assess adherence to conspiracy ideation about vaccines. This scale has not been validated to the Portuguese setting. This work intended to explore the applicability of the VCBS in this population, study conspiracy beliefs and its association with behaviours/attitudes towards vaccination against COVID-19. A cross-sectional study was conducted, using data from a representative sample of the national population (“At home we observe health” - ECOS panel). Reliability and structural validity of the scale were assessed, using confirmatory factor analysis. Five outcome variables were defined, regarding to behaviours/attitudes towards vaccination against COVID-19. We estimated odds ratios adjusted (ORa) for sociodemographic variables, and corresponding 95% confidence intervals (95%CI), through logistic regression. We included 767 individuals. The VCBS scale showed good internal consistency (Cronbach's alpha value=0.912). Principal component analysis identified only one component, explaining 66.4% of the variance. The average VCBS score was 17.4 (95%CI=16.24-18.56). Items with the highest scores referred to conspiracy theories about the pharmaceutical industry and the safety/security of vaccines. Negative associations were observed between VCBS scores and two outcome variables: taking the vaccine (ORa=0,958; 95%CI=0,925-0,992; p<0,05) and vaccination adherence (ORa=0,916; 95%CI=0,872-0,961; p<0,001). A positive association was observed between VCBS scored values and vaccine hesitancy (ORa=1,044; 95%CI=1,008-1,081; p<0,05). VCBS has shown to be a unidimensional scale, have high internal consistency and good structural validity. The level of conspiracy theories in Portugal seems to be lower than in other countries. Higher values of conspiracy beliefs were associated with lower vaccine intake and with a lower adherence to vaccination, but associated with greater vaccine hesitancy. This study may contribute to better strategies and policies directed at tackling conspiracy theories and aimed at achieving high levels of vaccination.