Objectives: To explore the perspectives of librarians and information specialists (LIS) on their experience and impact as peer reviewers of systematic reviews (SRs), and on facilitators and barriers to LIS methodological peer review. Design: Survey and focus groups. Setting: We surveyed LIS who completed a peer review of an SR in a randomised controlled trial conducted in BMJ, BMJ Open and BMJ Medicine from 3 January 2023 to 2 January 2024. The questionnaire sought to understand their experience, what aspects of manuscripts they focused on, perceived impact on editorial decision-making and authors’ revisions and willingness to peer review again. To better understand factors that might impact decisions to review again, we contacted survey respondents to participate in a focus group concentrating on facilitators and barriers to peer reviewing SRs. Participants: 88 LIS were eligible for participation. From the survey respondents, 27 LIS who had volunteered were randomly selected and invited to participate in a follow-up focus group. Results: Of the 88 LIS invited to participate in the survey, 70 (80%) responded. Most respondents had six or more years of experience as an LIS (67/70; 96%) and advising researchers on doing SRs (55/70; 79%) and had peer reviewed for a journal prior to the study (57/70; 81%). Most focused on the search and SR methods when reviewing but also commented on aspects such as research question formulation, plagiarism, study results and conclusions. Two-thirds (44/66; 67%) believed they impacted editors’ decision-making and 59% (39/66) believed they impacted the authors’ revisions. Only three factors were considered extremely or very likely to impact their decision to review again: their schedule and/or lack of time, review turnaround time and their sense of professional duty. 17 LIS (63.0%) participated in a focus group. Time was the primary barrier identified in the focus groups, followed by a sense of intimidation. LIS reported being motivated by feeling valued by editors, the enjoyment of peer reviewing, the desire to improve SR quality and peer review as a learning experience. Several expressed surprise and delight at being asked to peer review for the journals. Conclusions: LIS may be an underused peer reviewing resource with methodological experience that can help editors make decisions and improve the quality of SRs. Efforts to engage LIS as peer reviewers by journal editors are likely to be well-received when LIS expertise is clearly valued, sought and heeded. We encourage both journal editors and LIS to creatively advance efforts to promote LIS as methodological peer reviewers.