The gigabyte () is a multiple of the unit byte for digital information. The prefix giga means 10⁹ in the International System of Units (SI). Therefore, one gigabyte is one billion bytes. The unit symbol for the gigabyte is GB.

This definition is used in all contexts of science (especially data science), engineering, business, and many areas of computing, including storage capacities of hard drives, solid state drives, and tapes, as well as data transmission speeds. However, the term is also used in some fields of computer science and information technology to denote 1073741824 (1024³ or 2³⁰) bytes, particularly for sizes of RAM. Thus, prior to 1998, some usage of gigabyte has been ambiguous. To resolve this difficulty, IEC 80000-13 clarifies that a gigabyte (GB) is 10⁹ bytes and specifies the term gibibyte (GiB) to denote 2³⁰ bytes. These differences are still readily seen for example, when a 400 GB drive's capacity is displayed by Microsoft Windows as 372 GB instead of 372 GiB. Analogously, a memory module that is labeled as having the size "1GB" has one gibibyte (1GiB) of storage capacity.

In response to litigation over whether the makers of electronic storage devices must conform to Microsoft Windows' use of a binary definition of "GB" instead of the metric/decimal definition, the United States District Court for the Northern District of California rejected that argument, ruling that "the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce.'"