2.5G, which stands for "second and a half generation," is a cellular wireless technology developed in between its predecessor, 2G, and its successor, 3G.

"2.5G" is an informal term, invented solely for marketing purposes, unlike "2G" or "3G" which are officially defined standards based on those defined by the International Telecommunication (ITU). The term "2.5G" usually describes a 2G cellular system combined with General Packet Radio Services (GPRS), or other services not generally found in 2G or 1G networks.

Wireless telecommunication technology like CDMA200 1x-RTT, Enhanced Data Rates for GSM Evolution (EDGE) or Enhanced General Packet Radio Service (EGPRS), since they have data transmission rates of 144 kbps or higher, may qualify as 3G technology. However, they are usually classified as 2.5G technology because they have slower network speeds than most 3G services.

GPRS is a service commonly associated with 2.5G technology. It has data transmission rates of 28 kbps or higher. GPRS came after the development of the Global System for Mobile (GSM) service, which is classified as 2G technology, and it was succeeded by the development of the Universal Mobile Telecommunication Service (UMTS), which is classified as 3G technology.

A 2.5G system may make use of 2G system infrastructure, but it implements a packet-switched network domain in addition to a circuit-switched domain. This does not necessarily give 2.5G an advantage over 2G in terms of network speed, because bundling of timeslots is also used for circuit-switched data services (HSCSD).

The services and infrastructure of a 2.5G network may be used on a per-transaction basis rather than a per-minute-of-use basis, thanks to its packet-switched domain. This makes its infrastructure more efficient and improves the service delivery. This impetus is known as the "always-on" capability.

2.5G networks may support services such as WAP, MMS, SMS mobile games, and search and directory.