The Bath Ankylosing Spondylitis Activity Index (BASDAI), Functional Index (BASFI), Metrology Index (BASMI), and Global Score (BASG) are commonly used to assess patients with ankylosing spondylitis (AS). The aim of this study was to cross-culturally adapt and validate these indexes into the Portuguese language. Seventy-eight patients were included in the study. After forward and backward translations, the questionnaires were administered and tested for internal consistency, test-retest reliability, face validity, content validity, and construct validity. The outcome measures HAQ, EQ-5D, and SF-36 were also implemented. Metrological parameters (BASMI components) and chest expansion were evaluated. Correlation coefficients for test-retest were 0.875, 0.937, 0.831, and 0.961 for BASDAI, BASFI, BASMI, and BASG, respectively. Internal consistency coefficients were between 0.747 and 0.953. The adapted and translated questionnaires demonstrated an acceptable comprehensibility by a panel of patients, and face validity was assured by the cognitive debriefing performed. Content validity was assured by comparing the scores obtained by the questionnaires when age and gender, age of symptoms onset, and disease duration were considered. Construct validity was assured by significant correlations established between the Bath scores and generic health status HAQ, EQ-5D and SF-36, morning stiffness duration, chest expansion, and physician disease activity assessment. The Portuguese version of the BASDAI, BASFI, BASG, and BASMI showed adequate reliability and validity in patients with AS. The measurement properties were comparable to versions in other languages, indicating that the indexes can be used for evaluation of Portuguese-speaking AS patients.