Cognitive interference theories (e.g. attentional control theory, processing efficiency theory) suggest that high levels of trait anxiety predict adverse effects on the performance of cognitive tasks, particularly those that make high demands on cognitive resources. We tested an interaction hypothesis to determine whether a combination of high anxiety and low working memory capacity (WMC) would predict variance in demanding cognitive test scores. Ninety six adolescents (12- to 14-years-old) participated in the study, which measured self-report levels of trait anxiety, working memory, and cognitive test performance. As hypothesized, we found that the anxiety-WMC interaction explained a significant amount of variance in cognitive test performance (ΔR2 .07, p < .01). Trait anxiety was unrelated to cognitive test performance for those adolescents with average WMC scores (β = .13, p > .10). In contrast, trait anxiety was negatively related to test performance in adolescents with low WMC (β = −.35, p < .05) and positively related to test performance in those with high WMC (β = .49, p < .01). The results of this study suggest that WMC moderates the relationship between anxiety and cognitive test performance and may be a determinant factor in explaining some discrepancies found in the literature. Further research is needed to fully understand the mechanisms involved.