This study is the first to scrutinize the psychological effects of online astroturfing in the context of Russia’s digitally enabled foreign propaganda. Online astroturfing is a communicative strategy that uses websites, “sock puppets,” or social bots to create the false impression that a particular opinion has widespread public support. We exposed N = 2353 subjects to pro-Russian astroturfing comments and tested: (1) their effects on political opinions and opinion certainty and (2) the efficiency of three inoculation strategies to prevent these effects. All effects were investigated across three issues and from a short- and long-term perspective. Results show that astroturfing comments can indeed alter recipients’ opinions, and increase uncertainty, even when subjects are inoculated before exposure. We found exclusively short-term effects of only one inoculation strategy (refutational-same). As these findings imply, preemptive media literacy campaigns should deploy (1) continuous rather than one-time efforts and (2) issue specific rather than abstract inoculation messages.