TransCanada's wholly-owned subsidiary, TC PipeLines GP, Inc, is the general partner of TC PipeLines, LP, a United States growth-oriented Master Limited Partnership. TC PipeLines, LP was formed by TransCanada to acquire, own and actively participate in the management of U.S.-based natural gas pipelines and related assets. TC PipeLines GP, Inc. also holds common units of TC PipeLines, LP.

TC PipeLines, LP owns a 50 per cent interest in Northern Border Pipeline Company, which owns a 1,249-mile interstate pipeline system that transports natural gas from the Montana-Saskatchewan border to markets in the midwestern United States. It also owns or controls a 99 per cent interest in Tuscarora Gas Transmission Company, which owns a 240-mile interstate pipeline system that transports natural gas from Oregon, where it interconnects with TransCanada's GTN System, to northern Nevada. The third investment is a 46.45 per cent interest in Great Lakes Gas Transmission Limited Partnership, which owns a 2,115-mile interstate pipeline system that transports gas from Emerson on the Manitoba-Minnesota border to St. Clair on the Michigan-Ontario border. The fourth investment is 100% ownership of North Baja Pipeline, LLC which owns an 80-mile natural gas pipeline system extending from Ehrenberg, Arizona to a point near Ogilby, California and connects with the Gasoducto Bajanorte natural gas pipeline system in Mexico.

Common units of TC PipeLines, LP are quoted on the Nasdaq Stock Market and trade under the symbol TCLP.