The therapeutic value of antiviral nucleoside analogs was highlighted during the coronavirus disease 2019 (COVID-19) pandemic, with remdesivir and molnupiravir repurposed for their broad-spectrum antiviral activity. The cytidine analog azvudine (FNC) has recently gained attention as a potential treatment for human immunodeficiency virus type 1 (HIV-1) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Considering the distinct substrate specificities of HIV-1 reverse transcriptase (RT) and SARS-CoV-2 RNA-dependent RNA polymerase (RdRp), a unifying mechanism of inhibition remains elusive. Here, we assessed the inhibitory effects of FNC’s active triphosphate form, FNC-TP, across several viral polymerases. The relative efficiency of FNC-TP incorporation followed the order: HIV-1 RT > hepatitis C virus (HCV) RdRp > respiratory syncytial virus (RSV) RdRp > dengue virus type 2 (DENV-2) RdRp ≫ SARS-CoV-2 RdRp. Its incorporation caused chain-termination in all polymerases tested. Antiviral activity against HIV-1 has previously been demonstrated and is here shown with DENV-2. Collectively, the data show that inhibition of viral polymerases by FNC-TP can translate to antiviral activity against both retroviruses and RNA viruses, but the link is not evident for SARS-CoV-2. FNC-TP is a poor substrate for SARS-CoV-2 RdRp, and FNC lacks significant antiviral activity against SARS-CoV-2 in cell culture.