Pneumonia is a leading cause of morbidity and mortality worldwide, with community-acquired pneumonia (CAP) and hospital-acquired pneumonia (HAP) representing the two most common subtypes. Antimicrobials are central to treatment; however, systematic reviews (SRs) evaluating their use have reported highly variable outcomes, limiting evidence synthesis and comparability. The development of a core outcome set (COS) may address this gap.To summarize the primary outcomes reported in SRs of antimicrobials for adult pneumonia and to construct an preliminary list of candidate outcomes to inform COS development.A systematic search was performed in PubMed, Embase, Cochrane Library, CNKI, Wanfang Data, and China Science and Technology Journal Database from inception to December 2024. Eligible studies included SRs or meta-analyses that evaluated antimicrobial therapy in adults (≥ 18 years) with CAP or HAP. Two reviewers independently screened the studies, extracted data on the study characteristics, pneumonia type, severity assessment tools, and outcomes. The Corrected Covered Area (CCA) was calculated to quantify the overlap in primary outcome reporting. Outcomes were grouped by pneumonia type and severity and categorized into five domains following the COMET taxonomy.A total of 97 SRs were included. Twenty-one distinct primary outcomes were identified, with limited overlap. Mortality and clinical success were the most frequently reported outcomes, though inconsistently defined across studies. Significant variability was noted in the definitions and time points of outcomes, as well as in severity assessment tools. Registered SRs reported fewer outcomes than non-registered ones.There is considerable variation in primary outcome reporting in antimicrobial SRs for adult pneumonia. Mortality and clinical success are the most commonly reported outcomes, but their definitions lack consistency. Developing subtype- and severity-specific COS is crucial for standardizing outcome reporting and improving evidence synthesis.© 2025. The Author(s), under exclusive licence to Springer Nature Switzerland AG.