The blaze caused significant damage to the building's front entrance and a vehicle parked outside, though no one was injured in the incident.
Sussex Police have launched a fast-moving investigation, appealing for witnesses and any relevant footage, and have increased patrols in the area and around other places of worship.
Detective Superintendent Karrie Bohanna stated that Sussex Police have a zero-tolerance approach to hate crime and are working with local leaders to support the Muslim community.
The incident has been widely condemned by local politicians, including Labour MP Chris Ward and Lewes District Council leader Councillor Zoe Nicholson, who expressed solidarity with the affected community.