TC PipeLines, a TransCanada-affiliated partnership, agreed to acquire stakes in TransCanada's 1,353-mile Gas Transmission Northwest pipeline network and the 303-mile Bison natural gas pipeline system for around $1 billion.
The company said the acquisition increases its interest in the pipeline systems from 25 percent to 70 percent. TransCanada retains the rest.
"This acquisition is the largest transaction in the partnership's history and significantly increases the partnership's future cash flows and earnings," TC PipeLines President Steve Becker said in a statement.
TransCanada owns 33 percent of TC Pipelines, formed to take on interests in U.S. natural gas pipeline systems.
The Gas Transmission Northwest runs from Western Canada to markets on the western U.S. coast. Bison services the Rocky Mountain region.
TransCanada is waiting for U.S. government's approval of the $7 billion Keystone XL pipeline, a controversial project designed to carry so-called oil sands to southern U.S. refineries.
"The transaction demonstrates one of the many funding options available to TransCanada to finance our current capital commitments," TransCanada President Russ Girling said in a statement.