Rapid reviews (RRs) are a helpful evidence synthesis tool to support urgent and emergent decision-making in healthcare. RRs involve abbreviating systematic review methods and are conducted in a condensed timeline to meet the decision-making needs of organisations or groups that commission them. Knowledge users (KUs) are those individuals, typically patient and public partners, healthcare providers, and policy-makers, who are likely to use evidence from research, including RRs, to make informed decisions about health policies, programmes or practices. However, research suggests that KU involvement in RRs is often limited or overlooked, and few RRs include patients as KUs. Existing RR methods guidance advocates involving KUs but lacks detailed steps on how and when to do so. This paper discusses the importance of involving KUs in RRs, including patient and public involvement to ensure RRs are fit for purpose and relevant for decision-making. Opportunities to involve KUs in planning, conduct and knowledge translation of RRs are outlined. Further, this paper describes various modes of engaging KUs during the review lifecycle; key considerations researchers should be mindful of when involving distinct KU groups; and an exemplar case study demonstrating substantive involvement of patient partners and the public in developing RRs. Although involving KUs requires time, resources and expertise, researchers should strive to balance 'rapid' with meaningful KU involvement in RRs. This paper is the first in a series led by the Cochrane Rapid Reviews Methods Group to further guide general RR methods.