INTRODUCTION: Doxycycline postexposure prophylaxis (doxy-PEP) can prevent sexually transmitted infections (STIs) among men who have sex with men (MSM) and transgender women (TGW). STI rates are high among MSM and TGW in China, and implementation strategies are needed to optimise doxy-PEP services. Pay-it-forward and social network distribution approaches may increase uptake of STI services and could increase the uptake of doxy-PEP. We present the protocol for a randomised controlled trial evaluating the effectiveness of pay-it-forward strategies with and without adjunctive social network distribution among MSM and TGW in China. METHODS AND ANALYSIS: A total of 399 MSM and TGW will be recruited at seven sites in China and randomly allocated in a 1:1:1 ratio to (1) self-pay, (2) pay-it-forward alone or (3) pay-it-forward with adjunctive social network distribution of doxy-PEP. Participants assigned to the self-pay arm can purchase a doxy-PEP packet out-of-pocket. Participants in the pay-it-forward arm will be offered a free doxy-PEP packet and the opportunity to donate to support doxy-PEP for future participants. Participants in the pay-it-forward arm with social network distribution will receive the pay-it-forward intervention as well as additional free doxy-PEP packets to distribute to peers. Those randomised to the self-pay and the pay-it-forward with social network distribution arms (ie, index participants) will receive and distribute referral cards to recruit additional peers (ie, alter participants). Alter participants recruited through the control arm will be referred to the clinic to purchase doxy-PEP. Alter participants recruited through the pay-it-forward with adjunctive social network distribution arm will receive doxy-PEP directly from referring index participants. Both index and alter participants in each arm will be asked to complete a follow-up survey 3 and 6 months after enrolment. The primary outcome will be the proportion of participants who report using doxy-PEP within 72-hours of condomless anal or oral sex on one or more occasions during follow-up. ETHICS AND DISSEMINATION: Ethical approval was obtained from the ethics review committee of the Dermatology Hospital of Southern Medical University (Approval number: 2023109). The findings will be disseminated in peer-reviewed publications. TRIAL REGISTRATION NUMBER: The study has been registered with the Chinese Clinical Trial Registry (trial ID ChiCTR2300074903). Date of registration: 18 August 2023.