Natural product lead compounds from various marine sources have often been found to be promising pharmaceutical agents. Several of them have successfully been approved as antiviral agents for clinical use or have been advanced to the late stages of clinical trials. Most of these drugs are used for the treatment of Human Immunodeficiency Virus (HIV) and Herpes Simplex Virus (HSV). The most important antiviral lead of marine origin reported thus far is nucleoside Ara-A (vidarabine) isolated from sponge Tethya crypta. It inhibits viral DNA polymerase and DNA synthesis of herpes, varicella zoster viruses. However due to the discovery of new types of viruses and emergence of drug resistant strains, it is necessary to develop new antiviral lead compounds continuously. Over the past decade, several new experimental antiviral agents derived from marine sources have entered preclinical and clinical trials. In this review, a few examples of marine-derived experimental agents that are currently undergoing preclinical evaluation are briefly discussed.