Yes, there is. It is a decentralized network of local affiliates that work together using the internet and other mass communications methods.
Antifa adopted this structure deliberately to avoid the FBI and other law enforcement from being able to easily identify leadership and even members of the various local chapters. They learned this was necessary from earlier, traditionally organized radical Leftist groups that got infiltrated by law enforcement.
Antifa uses the dark web and sites like this one to organize and coordinate their actions.
In Search of New Forms of Life
itsgoingdown.org
Many local Antifa chapters do have their own webpages, Facebook accounts, etc., as well. They generally exercise considerable control over access to these having a public and private side. The real nasty stuff they do is on the dark web.