Dengue fever, caused by dengue virus (DENV) is the most prevalent arthropod-borne viral disease, and is endemic in many tropical and sub-tropical parts of the world with an increasing incidence in temperate regions. The closely related flavivirus Zika virus (ZIKV) can be transmitted vertically and causes congenital Zika syndrome and other birth defects. In adults, ZIKV is associated with Guillain-Barré syndrome. There are no approved antiviral therapies against neither viruses. Effective antiviral compounds are urgently needed. Amaryllidaceae alkaloids (AAs) are a specific class of nitrogen-containing compounds produced by plants of the Amaryllidaceae family with numerous biological activities. Recently, the AA lycorine was shown to present strong antiflaviviral properties. Previously, we demonstrated that contained lycorine and several alkaloids of cherylline, crinine and galanthamine-types with unknown antiviral potential. In this study, we explored their biological activities. We show that crude alkaloid extract inhibited DENV infection. Among the purified AAs, cherylline inhibited efficiently both DENV (EC=8.8 μM) and ZIKV replication (EC=20.3 μM), but had no effect on HIV-1 infection. Time-of-drug-addition and -removal experiments identified a post-entry step as the one targeted by cherylline. Consistently, using subgenomic replicons and replication-defective genomes, we demonstrate that cherylline specifically hinders the viral RNA-synthesis step but not viral translation. In conclusion, AAs are an underestimated source of antiflavivirus compounds, including the effective inhibitor cherylline that could be optimized for new therapeutic approaches.