The efficacy of silver nanoparticles (AgNPs) was tested in vitro against three different fish viruses, causing significant economic damage in aquaculture. These viruses were the spring viraemia of carp virus (SVCV), European catfish virus (ECV), and Ictalurid herpesvirus 2 (IcHV-2). The safe concentration of AgNPs that did not cause cytotoxic effects in EPC cells proved to be 25 ng/mL. This dose of AgNPs decreased significantly (5–330×) the viral load of all three viruses in three different types of treatments (virus pre-treatment, cell pre-treatment, and cell post-treatment with the AgNPs). In a higher concentration, the AgNPs proved to be efficient against ECV and IcHV-2 even in a delayed post-cell-treatment experiment (AgNP treatment was applied 24 h after the virus inoculation). These first in vitro results against three devastating fish viruses are encouraging to continue the study of the applicability of AgNPs in aquaculture in the future.