In computer hardware a host controller, host adapter or host bus adapter (HBA) connects a computer system bus which acts as the host system to other network and storage devices. The terms are primarily used to refer to devices for connecting SCSI, SAS, NVMe, Fibre Channel and SATA devices. Devices for connecting to FireWire, USB and other devices may also be called host controllers or host adapters.

Host adapters can be integrated in the motherboard or be on a separate expansion card.

The term network interface controller (NIC) is more often used for devices connecting to computer networks, while the term converged network adapter can be applied when protocols such as iSCSI or Fibre Channel over Ethernet allow storage and network functionality over the same physical connection.

A SCSI host adapter connects a host system and a peripheral SCSI device or storage system. These adapters manage service and task communication between the host and target. Typically a device driver, linked to the operating system, controls the host adapter itself.

In a typical parallel 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 SCSI 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.

Major SCSI adapter manufacturers are HP, ATTO Technology, Promise Technology, Adaptec, and LSI Corporation. LSI, Adaptec, and ATTO offer PCIe SCSI adapters which fit in Apple Mac, on Intel PCs, and low-profile motherboards which lack SCSI support due to the inclusion of SAS and/or SATA connectivity.