Pronounced difficulties in functional outcomes often follow acquired brain injury (ABI), and may be due, in part, to deficits in metacognitive knowledge (being unaware of one's cognitive strengths and limitations). A meta-analytic review of the literature investigating the relationship between metacognitive knowledge and functional outcomes in ABI is timely, particularly given the presence of apparently inconsistent findings. Twenty-two articles revealed two distinct methods of measuring metacognitive knowledge: (1) absolute (the degree of inaccurate self-appraisal regardless of whether the error tends towards under- or over-confident estimations) and (2) relative (the degree and the direction of the inaccuracy) discrepancy. Separate meta-analyses were conducted for absolute and relative discrepancy studies to assess the relationship between metacognitive knowledge and functional outcomes (affect-related quality of life, family and community integration, and work outcomes). The pattern of results found suggested that better metacognitive knowledge is related to better overall functional outcomes, but the relationship may differ depending on the outcome domain. These findings generally support the importance of focusing on metacognitive knowledge to improve outcomes following ABI. Nonetheless, the relatively small effect sizes observed suggest that other predictors of functional outcome should be investigated, including other subdomains of metacognition.