In computer hardware a host adapter, as an interface device, connects a SCSI bus to a computer (the host). The host adapter bridges the physical and logical chasm that separates the SCSI bus from the host computer's internal bus. Modern host adapters contain all the electronics and firmware required to execute SCSI transactions, and often include a BIOS that not only allows the host system to boot from a SCSI device, but also facilitates configuration of the host adapter. Typically a device driver, linked to the operating system, controls the host adapter itself.

In a typical SCSI subsystem, each device has assigned to it a unique numerical ID. As a rule, the host adapter appears as SCSI ID 7, which gives it the highest priority on the SCSI bus (priority descends as the SCSI ID descends; on a 16-bit or "wide" bus, ID 8 has the lowest priority, a feature that maintains compatibility with the priority scheme of the 8-bit or "narrow" bus).

The host adapter usually assumes the role of "initiator," in that it issues commands to other SCSI devices.

A computer can contain more than one host adapter, which can greatly increase the number of SCSI devices available.