The Transatlantic Trade and Investment Partnership (TTIP) negotiations between the EU and US were highly technical, yet provoked widespread protests with diverse socioeconomic backgrounds. We contend that transparency in these complex processes was a key factor driving public mobilization. Using survey experiments, we demonstrate a causal link: non-transparent decision-making significantly reduces citizens' appraisal of trade agreements independently of their economic consequences. Specifically, our findings reveal that transparent negotiations increase the likelihood of receiving public approval by nearly 16 percent compared to identical opaque agreements. This research underscores the importance of transparency in democratic policy evaluation processes.