Union is a station on the Yonge-University Spadina line of the Toronto subway. It is placed at the bottom of the 'loop' formed by that line, between the Yonge and University sections, at its southernmost point. It is located at 55 Front Street West between Bay Street and York Street below Union Station. It opened in 1954. Union station offers connection to the GO Transit system, VIA Rail, Ontario Northland, and Amtrak. Nearby landmarks include Union Station, Royal York Hotel, the Air Canada Centre, the Toronto Convention Centre , the CN Tower, the Royal Bank Plaza, BCE Place, and the Hockey Hall of Fame.

TTC bus and streetcar routes connecting to Union:

• 6 Bay
• 65 Parliament
• 72 Pape
• 97 Yonge
• 503 Kingston Rd streetcar
• 509 Harbourfront
• 510 Spadina

An underground streetcar loop was added to the station in 1990 and is now served by the 509 Harbourfront and 510 Spadina routes. Union's status as a transport hub has placed a lot of strain on its narrow centre platform. A plan is in place to add a new platform for eastbound trains, outside the tracks; the old platform will then only need to serve westbound trains.

Leaving the station eastbound, the Yonge leg of the line runs briefly along Front Street and turns 90 degrees north to run under Yonge Street; leaving westbound, the University-Spadina leg also runs along Front and eventually turns 90 degress north to run under University Avenue.

Union was the southern terminus of the original 1954 Yonge subway line.