Rationale: The public should be informed about overtesting and overdiagnosis. Diverse qualitative studies have examined public understandings of this information. A synthesis was needed to systematise the body of evidence and yield new, generalisable insights. Aim: Synthesise data from qualitative studies exploring patient and public understanding of overtesting and overdiagnosis. Methods: We searched Scopus, CINAHL, Ovid MEDLINE and PsycINFO databases from inception to March 18, 2020. We included published English-language primary studies exploring the perspectives of patients/the public about overtesting/overdiagnosis from any setting, year and relating to any condition. Only qualitative parts of mixed-methods studies were synthesised. We excluded studies that only examined overtreatment or sampled people with specialised medical knowledge. Two authors independently selected studies, extracted data, assessed the methodological quality of included studies using the CASP tool, and assessed confidence in the synthesis findings using the GRADE-CERQual approach. Data was analysed using thematic meta-synthesis, utilising descriptive and interpretive methods. Results: We synthesised data from 21 studies, comprising 1638 participants, from 2754 unique records identified. We identified six descriptive themes, all graded as moderate confidence (indicating they are likely to reasonably represent the available evidence): i) high confidence in screening and testing; ii) difficulty in understanding overuse; iii) acceptance that overuse can be harmful; iv) rejection or problematisation of overuse; v) limited impacts of overuse information on intended test and screening uptake; vi) desire for information and shared decision-making regarding overuse. The descriptive themes were underpinned by two analytic themes: i) perceived intrinsic value of information and information gathering, and; ii) differences in comprehension and acceptance of overuse concepts. Conclusions: This study identified novel and important insights about how lay people interpret overuse concepts. It will guide the development of more effective public messages about overuse, highlighting the importance of interpretative frameworks in these communications.