Background: Many frequently used instruments fail to assess psychosocial functioning in patients with bipolar disorder. The Functioning Assessment Short Test (FAST) was developed in order to tackle this problem and to assess the main functioning problems experienced by patients with bipolar disorder. However, the original FAST is not fully applicable in older adults due to the domain of occupational functioning. The aim of our study was to validate an adapted version for Older adults (FAST-O) in a group of older adults with bipolar disorder (OABD). Methods: 88 patients aged 50 years and over diagnosed with bipolar disorder were included. We adapted the items in the area of “work-related functioning” of the FAST into items assessing “societal functioning”. Several measurements were conducted in order to analyse the psychometric qualities of the FAST-O (confirmatory factor analysis for internal structure, Cronbach’s alpha for internal consistency, Spearman’s rho for concurrent validity, Mann–Whitney U test for discriminant validity). Results: Mean age in the study sample was 65.3 (SD = 7.5) and 57.3% was female. The internal structure was most similar to the internal structure of the original FAST. The internal consistency was excellent (Cronbach’s alpha =.93). The concurrent validity when correlated with the Social and Occupational Functioning Assessment Scale was low, but significant. The FAST-O was also able to distinguish between euthymic and symptomatic OABD patients. Conclusions: The FAST-O has strong psychometric qualities. Based on our results, we can conclude that the FAST-O is a short, efficient solution in order to replace global rating scales or extensive test batteries in order to assess daily functioning of older psychiatric patients in a valid and reliable manner.