BACKGROUND: Regular chlamydia and gonorrhea testing are essential for key populations, such as female sex workers (FSWs). However, testing cost, stigma, and lack of access prevent FSWs in low- and middle-income countries from receiving chlamydia and gonorrhea testing. A social innovation to address these problems is "pay it forward," where an individual receives a gift (free testing) and then asks whether they would like to give a gift to another person in the community. OBJECTIVE: This cluster randomized controlled trial examined the effectiveness and cost of the pay-it-forward strategy in increasing access to chlamydia and gonorrhea testing among FSWs in China. METHODS: This trial integrated a pay-it-forward approach into a community-based HIV outreach service. FSWs (aged 18 years or older) were invited by an outreach team from 4 Chinese cities (clusters) to receive free HIV testing. The 4 clusters were randomized into 2 study arms in a 1:1 ratio: a pay-it-forward arm (offered chlamydia and gonorrhea testing as a gift) and a standard-of-care arm (out-of-pocket cost for testing: US $11). The primary outcome was chlamydia and gonorrhea test uptake, as ascertained by administrative records. We conducted an economic evaluation using a microcosting approach from a health provider perspective, reporting our results in US dollars (at 2021 exchange rates). RESULTS: Overall, 480 FSWs were recruited from 4 cities (120 per city). Most FSWs were aged ≥30 years (313/480, 65.2%), were married (283/480, 59%), had an annual income <US $9000 (301/480, 62.7%), and had never been tested for chlamydia (401/480, 83.5%) or gonorrhea (397/480, 82.7%). Chlamydia and gonorrhea test uptake in the pay-it-forward and standard-of-care arms were 82% (197/240) and 4% (10/240), respectively, with an adjusted proportion difference of 76.7% (lower bound 95% CI 70.8%). All those who tested positive were referred to and received treatment at local sexually transmitted infection clinics. This finding was consistent when adjusting for marital status, income, inconsistent condom use during commercial sex in the last 3 months, and HIV testing history. Among 197 women who received tests in the pay-it-forward arm, 99 (50.3%) donated money, with a median donation of US $1.54 (IQR 0.77-1.54). The economic cost per person tested was US $568.71 for standard of care and US $43.20 for pay it forward. CONCLUSIONS: The pay-it-forward strategy has the potential to enhance chlamydia and gonorrhea testing for Chinese FSWs and may be useful for scaling up preventive services. Further implementation research is needed to inform the transition of pay-it-forward research into practice. TRIAL REGISTRATION: Chinese Clinical Trial Registry ChiCTR2000037653; https://www.chictr.org.cn/showprojen.aspx?proj=57233.