Current uptake of the COVID-19 vaccine in the UK and US is only around 50%, and other countries, like Australia, are witnessing slow uptake. It is therefore important to identify the concerns and social cognitions that relate to vaccination intention and, potentially, uptake. To date, no theory-based measure captures sets of beliefs expected to relate to COVID-19 vaccination intentions. This study addresses this evidence gap by developing and validating a comprehensive measure that captures these concerns and cognitions, the Vaccination Concerns and Cognitions in COVID-19 Scale (VaCCS). We followed a systematic, multistep process to develop and validate the VaCCS. In the development phase, samples of Australian (N=53; MAge=44.45, SDAge=19.57, 36% female) and US (N=48; MAge=36.95, SDAge=12.64, 58% female) residents recruited via an online research panel company completed an initial open-response survey. Concurrent to the survey, a rapid review of the literature was conducted to identify relevant content from existing scales assessing vaccination beliefs. An initial pool of items was developed from the survey and rapid review content. This was followed by an assessment of the readability and face validity of the item pool by 5 experts comprising clinical and health psychologists, behaviour change scientists, and a vaccination researcher (4 females) and 10 non-experts (MAge=39.00, SDAge=18.86, 50% female). In the scale validation phase, samples of Australian (N=522, 60.7% female) and US (N=499, 70.7% female) residents via an online research panel company completed scaled versions of the final item pool and a battery of socio-political, health beliefs and outcomes, and trait measures. Exploratory factor analysis yielded a final scale comprising 35 items with eight subscales. Confirmatory factory analysis yielded acceptable fit of the 8-subscale structure in each sample and multi-sample analysis supported factorial invariance across samples. Concurrent and predictive validity tests indicated a theoretically and conceptually predictable pattern of relations between the VaCCS subscales with the socio-political, health beliefs and outcomes, and trait measures. The VaCCS provides a novel measure to assess concerns and social cognitions toward COVID-19 vaccination and facilitates future intervention design and evaluation for vaccination uptake in the current pandemic and beyond.