AREX (Airport Railroad Express; Korean: 인천국제공항철도) is an airport rail link and commuter rail line in the Seoul Metropolitan Area of South Korea, linking Incheon International Airport and Gimpo International Airport with Seoul Station. The section between the two airports opened on March 23, 2007, and the line was extended to Seoul Station on December 29, 2010.The color used for route information for regular trains is a shade of blue called 'sea blue', while for express trains, it is orange.

Long-distance Korea Train Express high speed trains started to use the line from June 30, 2014, but discontinued the service in March 2018 due to low ridership.

The line was initially announced in July 1998 as the Incheon International Airport Railroad (인천국제공항철도), abbreviated IREX (Incheon Airport Railroad Express) which can be seen on the railings of overhead crossings by the line. The project was launched as South Korea's first build-operate-transfer (BOT) franchise. The concession was won by a consortium of 11 Korean companies, which incorporated as the Incheon International Airport Railroad Company (IIARCo) in March 2001. The original main shareholders were Hyundai (27%), POSCO (11.9%), Daelim (10%), Dongbu (10%) and the Korean National Railroad (9.9%).

After significant delays, construction of the line started in 2001, after the opening of Incheon Airport. About 60% of the line is underground, and reaches Incheon International Airport on Yeongjong Island via the lower deck of Yeongjong Bridge, a combined road-rail bridge. IIARCo contracted project management and the supply of equipment and trains, altogether worth about €400 million, to Incheon Korean French Consortium (IKFC). This consortium includes French company Alstom, its South Korean subsidiary Eukorail, and the South Korean rolling stock manufacturing company Rotem, which also supplied the technology for the KTX high-speed rail system.

The line and the operating company was renamed AREX in June 2006. The first 37.6 km (23.4 mi) segment from Incheon International Airport Terminal 1 to Gimpo International Airport opened on March 23, 2007. This makes AREX the first dedicated airport rail link and the second railway line in the Korean Peninsula (after Seoul Subway Line 5) to directly serve an airport.

In March 2009, Korail acquired an 88.8% share of the company. Upon the acquisition, the operating company changed its name to Korail Airport Railroad as of November 30, 2009.

Phase 2 of the line runs from Gimpo International Airport to Seoul Station, which increased the length to 58 km (36 mi). At Gimpo the line is underground, rising to the surface to cross the Han River on the Magok Railway Bridge before descending underground for the final time. This section was built in a cut and cover tunnel running parallel to Susaek and Gajwa before connecting with Hongik University and Gongdeok, then diving to terminate deep underground on the west side of Seoul Station. The second phase opened for regular service on December 29, 2010, with the exception of Gongdeok station, which was planned to open around early 2011, but was delayed to November 29, 2011.

Phase 3 of the line was opened in September 2018. During phase 3 Magongnaru station was opened.