NEW YORK — Cogeco Cable Inc., a Montreal cable television company, agreed to buy Atlantic Broadband for $1.36 billion, letting it expand into the United States after a failed attempt to gain a foothold in Europe.

Cogeco is purchasing the company from private equity firms Abry Partners and Oak Hill Capital Partners in a deal expected to be completed by yearend, the Canadian company said Wednesday in a statement. Atlantic Broadband, based in Quincy, Mass., operates cable systems in Pennsylvania, Florida, Maryland, Delaware, and South Carolina.

For Cogeco, the deal raises investor concerns that the company is making another risky international gamble, sending the shares down as much as 17 percent.

Cogeco is pushing into the United States after abandoning the struggling European market, where it wrote off the value of its Portuguese acquisition Cabovisao-Televisao por Cabo SA and lost $248 million last year.

“The company has announced another surprise acquisition in a new operating territory,” Andrew Calder, an analyst at RBC Capital Markets in Toronto, said in a note to clients. “Investors will be hoping that the competitive conditions and execution prove to be more favorable this time around.”

The transaction is Cogeco’s biggest acquisition as a public company, more than double the size of its 2006 purchase of Cabovisao-Televisao for about $600 million.

Atlantic Broadband was formed in 2003 after it purchased cable systems from Charter Communications Inc. in St. Louis. It currently ranks as the 14th-largest US cable operator, according to the company’s website.

Atlantic Broadband serves about 252,000 basic video customers and provides cable, phone, and Internet to rural communities. That suits Cogeco’s experience of offering service in less-populated areas of Quebec and Ontario, chief executive Louis Audet said.

“There is room as you can see for further US growth, in contrast to Canada,” he said during a conference call Wednesday.

The purchase helps reduce Cogeco’s dependence on its current market, where it competes with larger cable operators Rogers Communications Inc., BCE Inc., Telus Corp., and Shaw Communications Inc.

“This is an attractive entry point into the United States,” Audet said. The deal also positions Cogeco to make further tuck-in acquisitions, he said.

Cable consolidation allows companies to avoid paying duplicate programming fees while combining their sales forces and paring infrastructure costs. Smaller operators are under pressure to merge because programming fees are climbing by about 7 percent to 10 percent a year, eating into their profit margins.