Alpha suppression is proposed to reflect a surge in cortical excitability to enhance stimulus processing in working memory. The attenuated state of alpha might reflect the prioritisation of behaviourally relevant information, making it a proxy for working memory functioning. Despite the growing interest in utilising the advancement of brain-based measures to evaluate individuals’ cognitive processes, there was a lack of consistent evidence on the relationship between alpha suppression and working memory performance. To investigate whether interindividual differences in alpha suppression might be related to variability in working memory capacity, we recorded participants’ electroencephalography (EEG) while they performed an arithmetic task of either low or high working memory load. Participants were required to calculate either the product of digits (i.e., low-load condition) or the difference between the product of digits (i.e., high-load condition). We found alpha suppression at parietal regions, which became more prominent as working memory load increased. The pattern was present in approximately 80% of the participants. Importantly, the more the alpha suppressed as working memory load increased, the larger the drops in behavioural performance and the lower the Digit Span score. That is, alpha suppression was more prominent in participants of poor working memory capacity. Our findings suggest that alpha activity, subject to interindividual differences in sensitivity, could serve as a brain-based measure of an individual’s working memory functioning.