Objective. Transparency of research methods is vital to science, though incentives are variable, with only some journals and funders adopting transparency policies. Clearinghouses are also important stakeholders; however, to date none have implemented formal procedures that facilitate transparent research. Using data from the longest standing clearinghouse, we examine transparency practices for preventive interventions to explore the role of online clearinghouses in incentivizing researchers to make their research more transparent. Methods. We conducted a descriptive analysis of 88 evaluation reports reviewed in 2018-19 by Blueprints for Healthy Youth Development, when the clearinghouse began checking for trial registrations, and expanded on these efforts by applying broader transparency standards to interventions eligible for an endorsement on the Blueprints website during the study period. Reports were recent, with 84% published between 2010 and 2019. Results. We found few reports had data, code, or research materials that were publicly available. Meanwhile, 40% had protocols that were registered, but only 8% were registered prospectively, while one-quarter were registered before conducting analyses. About one-third included details in a registered protocol describing the treatment contrast and planned inclusions, and less than 5% had a registered statistical analysis plan (e.g., planned analytical methods, pre-specified covariates, etc.). Confirmatory research was distinguished from exploratory work in roughly 40% of reports. Reports published more recently (after 2015) had higher rates of transparency. Conclusions. Preventive intervention research needs to be more transparent. Since clearinghouses rely on robust findings to make well-informed decisions and researchers are incentivized to meet clearinghouse standards, clearinghouses should consider policies that encourage transparency to improve the credibility of evidence-based interventions.