A seamless handoff system and method reduces delay and packet loss during handoff of user equipment (UE) from a Wireless Local Area Network (WLAN) network to a Universal Mobile Telecommunication System (UMTS) network. The handoff system and method performs pre-registration and pre-authentication processes before the occurrence of a handoff, buffers packets being delivered to a UE during a handoff period, and forwards the buffered packets to the UE after completion of the handoff, thereby reducing packet loss that can occur during handoff.