Research examining the joint relationships between test anxiety, metacognition, and mathematics achievement revealing the mediational role of metacognition in the relationship between test anxiety and mathematics achievement is sparse. A mediation study was designed to redress this imbalance. The Children’s Test Anxiety Scale (CTAS), Junior Metacognitive Awareness Inventory (Jr. MAI), and Mathematics Achievement Test (MAT) were distributed to 943 (442 males and 501 females) Grade 7 (n = 477) and Grade 8 (n = 466) students aged between 11–12 (Mage = 11.5, SD = .88) and 11–13 (Mage = 12, SD = .91) years, respectively. In this study, multiple mediation models were tested to explore the role of metacognition as a mediator of the effect of test anxiety on mathematics achievement. Results indicate that although both test anxiety and metacognition were significantly related to mathematics achievement, metacognition was not a statistically significant mediator in the relationship between test anxiety and mathematics achievement. Specifically, the knowledge of cognition component of metacognition was the only significant mediator, mediating the relationship between off-task behaviors and mathematics achievement. Findings support the beneficial role of metacognition with the rewarding side of a key implication that without developing metacognitive knowledge, efforts at alleviating test anxiety to maximize achievement in mathematics may well be fruitless. Educational and practical implications are discussed.