The Iranian government blocked Signal after local users migrated to the messaging platform following privacy concerns from WhatsApp.
The Observatory of Network Interference (OONI) first reported the block last January 25.
Users in Tehran and at least three other major cities reported that the block started around Monday noon.
Despite the block, the app could still be reportedly accessed with a VPN.
The Iranian government has not publicly commented on the decision to ban Signal but said earlier this month that the app allegedly contained criminal content and ordered its removal from app stores.
Signal announced that it is working to make Signal available again in Iran.
“Unable to stop registration, the IR censors are now dropping all Signal traffic. Iranian people deserve privacy. We haven’t given up,” said Signal on their Twitter account.