Introduction The use of child sexual abuse material (CSAM) is an international public health and child protection challenge. Objective To investigate whether Prevent It, a therapist-supported, internet-delivered, eight-week, cognitive behavioral therapy, reduces CSAM viewing among users. Methods We conducted a global online single-blind (participants), parallel-group, superiority, randomized, psychological placebo-controlled trial with a one-month follow-up, 2019–2021 (ISRCTN76841676). We recruited anonymous participants, mainly from Darknet forums. Inclusion criteria: age 18+ years, past week CSAM use, and sufficient English language skills; exclusion criteria: severe psychiatric illness or non-serious intent to participate. The main outcome was change in self-reported, weekly viewing time from pre- to post-treatment, according to the Sexual Child Molestation Risk Assessment+. Results A total of 160 participants (157 male, 2 non-binary, and 1 not reporting gender) from all world regions (age intervals [%]: 18–29 [49]; 30–39 [30]; 40–49 [15]; 50–59 [6]) were randomized (1:1) to Prevent It (N = 80) or Placebo (N = 80). Between-group, intention-to-treat analyses suggested a significantly larger decrease in viewing time in Prevent It participants vs. controls pre- to post-treatment (Prevent It: N = 76, Placebo: N = 78, estimate −0.25, 95 % CI, −0.46 to −0.04, p = .017, Cohen's d 0.18). Negative side effects from treatment were fewer in Prevent It compared to control participants and neither group reported severe adverse events. Conclusion We provide initial support for the feasibility, efficacy, and safety of Prevent It to reduce CSAM viewing among motivated users. Further research is needed to validate these findings.