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.

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