USB4 is a USB system specified in the USB4 specification which was released in version 1.0 on 29 August 2019 by USB Implementers Forum.

In contrast to prior USB protocol standards, USB4 requires USB-C connectors, and for power delivery, it requires support of USB PD. In contrast to USB 3.2, it allows tunneling of DisplayPort and PCI Express. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application. USB4 products must support 20 Gbit/s throughput and can support 40 Gbit/s throughput, but due to tunneling even nominal 20 Gbit/s can result in higher effective data rates in USB4, compared to USB 3.2, when sending mixed data.

The USB4 specification is based on the Thunderbolt 3 protocol specification. Support of interoperability with Thunderbolt 3 products is optional for USB4 hosts and USB4 peripheral devices, and is required for USB4 hubs on their downward facing ports and for USB4-based docks on their downward and upward facing ports.