Marek’s disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by an alphaherpesvirus, Marek’s disease virus (MDV). MD is presently controlled by systematic vaccination of animals, which protects efficiently against clinical disease. However, MDV vaccines do not prevent the multiplication and spread of MDV field strains generating immunosuppression. They may also favor the emergence of strains with increased virulence. Therefore, MDV persists to be a major problem for the poultry industry generating economic losses estimated at 1-2 billion dollars per year. The development of new sustainable alternative strategies to control MDV is needed. Macroalgae extracts have previously been shown to exert antiviral and immunomodulatory activities that could support the animals to better resist to this challenge. The objective of the study was to explore the effect of sulphated green macroalgae (Ulva sp.) extract (MSP®IMMUNITY) on MDV infection in vitro. To determine the impact on MDV lytic replication, chick embryo fibroblasts (CEFs) were infected and treated with increasing doses of the algae extract. The viral load was quantified at 24, 48, 72 and 96 hours post infection (hpi) by qPCR. The results showed the treatments significantly decreased MDV lytic replication in CEFs in a dose-dependent manner with the strongest effect observed with concentrations of 1ml/l. From 24 hpi, the viral load was reduced by about 80% with the algae extract at a concentration of 2ml/l and this is maintained over time and reached 94% at 96 hpi, A substantial decrease in MDV plaque size (from 2 to 3 fold) was demonstrated suggesting that macroalgae extract impede MDV cell-to-cell spread in vitro. This study provides the first evidence that the use of the macroalgae extracts could be a good alternative to limit MDV infection in poultry.