USB Attached SCSI (UAS) or USB Attached SCSI Protocol (UASP) is a computer protocol used to move data to and from USB storage devices such as hard drives (HDDs), solid-state drives (SSDs), and thumb drives. UAS depends on the USB protocol, and uses the standard SCSI command set. Use of UAS generally provides faster transfers compared to the older USB Mass Storage Bulk-Only Transport (BOT) drivers.

UAS was introduced as part of the USB 3.0 standard, but can also be used with devices complying with the slower USB 2.0 standard, assuming use of compatible hardware, firmware and drivers. UAS was developed to address the shortcomings of the original USB Mass Storage Bulk-Only Transport protocol, i.e., an inability to perform command queueing or out-of-order command completions. To support these features, the Bulk Streaming Protocol was added to the USB3 specification, and Streams support was added to the USB host controller interface (Extensible Host Controller Interface).

UAS is defined across two standards, the T10 "USB Attached SCSI" (T10/2095-D) referred to as the "UAS" specification, and the USB "Universal Serial Bus Mass Storage Class – USB Attached SCSI Protocol (UASP)" specification. The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the UAS specification; the SCSI Trade Association (SCSITA) promotes the UAS technology. The USB mass-storage device class (MSC) Working Group develops and maintains the UASP specification; the USB Implementers Forum, Inc. (USB-IF) promotes the UASP technology.

UAS drivers generally provide faster transfers when compared to the older USB Mass Storage Bulk-Only Transport (BOT) protocol drivers. Although UAS was added in the USB 3.0 standard, it can also be used at USB 2.0 speeds, assuming compatible hardware.

When used with an SSD, UAS is considerably faster than BOT for random reads and writes given the same USB transfer rate. The speed of a native SATA 3 interface is 6.0 Gbit/s. When using a USB 3.0 link (5.0 Gbit/s), which is slower than a SATA 3 link, the performance is limited by the USB link. However, later USB protocols have higher transfer rates, with USB4 allowing 80 Gbit/s. A UAS drive can be implemented using a SATA 3 drive attached through a SATA–UAS bridge with the SATA transfer rate limiting throughput, however, a native UAS SSD can take full advantage of higher USB transfer rates.

The original UAS standard (ANSI INCITS 471-2010 and ISO/IEC 14776-251:2014) can be referred to as UAS-1. A UAS-2 project was started by T10 but cancelled. That effort was resurrected as UAS-3, which is now a published standard (INCITS 572-2021). Apart from being based on later versions of other SCSI standards (e.g. SAM-6 and SPC-6, both under development) the technical author described the changes between UAS-1 and UAS-3 as follows: "allow the device to switch data transfers from one command to another before the current command is complete".

In July 2010 SemiAccurate reported that Gigabyte Technology had introduced working UAS drivers for their hardware using NEC/Renesas chips.

A comparative performance review by VR-Zone in August 2011 concluded that only the NEC/Renesas chips had working UAS drivers. The same Renesas UAS driver (for Windows) also works with AMD's A70M and A75 Fusion Controller Hubs, the USB part of which was co-developed by AMD and Renesas. In October 2011, ASMedia USB controllers chips had gained driver support as well (they had support on the hardware side before).