Within research and policy on preventing and countering terrorism, transparency is viewed as a necessity to generate public support and trust for counter-terrorism policies. Yet there is no systematic evidence to support these assumptions while research in other policy areas has challenged these assumptions, showing some forms of transparency might decrease support and trust. This paper presents results from two experimental surveys conducted in the United Kingdom to examine the effect of increased transparency on support and trust for terrorism prevention policy. Our findings challenge the widely held assumptions with regard counter-terrorism policy: increased policy information about a prevention policy (based on real Prevent websites) decreases support for Prevent, it makes people less likely to report suspected radicalisation to Prevent, and it has no effect on trust. Conversely, transparency which communicates the rationale behind policy decisions (in this case, the controversial Prevent referral process) increases policy acceptance, decreases the intent to protest, and increases trust in the prevention programme. The findings have global implications for counter-terrorism policy which is primarily based on positive, linear assumption on the relationship between transparency, trust and support—the most common form of transparency these policies use is at best ineffective and at worse counter-productive.