The deal includes $188m in cash along with the assumption of a proportionate share of PNGTS debt amounting to $35m.
As per the deal, TC PipeLines will make additional payment to TransCanada if the PNGTS is expanded in the future.
TC PipeLines president Steve Becker said: "This transaction is another in a series of expected dropdowns from TransCanada and again demonstrates our strategy of investing in assets that provide stability and long-term value to our unitholders."
TransCanada expects the sale to have earnings before interest, taxes, depreciation and amortisation (EBITDA) of about $23m in 2016.
TransCanada president and CEO Russ Girling said: "The PNGTS transaction is a continuation of our strategy to drop down the remainder of TransCanada’s US natural gas pipeline assets to the TC PipeLines partnership.
"Asset sales to the partnership provide TransCanada with cash proceeds to help fund our capital programme and further diversify the partnership’s asset base, positioning it for continued growth."
Upon completion of the transaction, TransCanada will continue to hold a direct 11.8% interest in the pipeline.
The sale, which is subject to various conditions, is planned to be completed by the end of 2015.
Commissioned in March 1999, the 474km, high-pressure interstate natural gas pipeline PNGTS is designed to connect the TransQuébec and Maritimes Pipeline at the Canadian border near East Hereford, Québec to deliver natural gas to customers in the US northeast.
The project also shares facilities with the Maritimes and Northeast Pipeline from Westbrook, Maine to a connection with the Tennessee Gas Pipeline System, which is located near Boston, Massachusetts.
