John T. Mather Memorial Hospital in Port Jefferson on Thursday said it has signed a letter of intent to join Northwell Health.

Mather would become Northwell’s fifth hospital in Suffolk County.

The hospital said it has held discussions with several health systems regarding a possible affiliation since last year. Mather said it considered joining Mt. Sinai Health System, Stony Brook Medicine and Catholic Health Services before choosing Northwell.

Over the next few months, Mather and Northwell Health will work to develop a final agreement, which will require approval from both boards, as well as regulatory authorities.

The deal would close by the end of 2017, according to Mather.

Mather currently has 248 beds, handles 12,000 inpatients and 41,000 emergency room cases each year. Mather has 2,568 employees, according to its website.

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When the nearly 88-year-old hospital opened in December 1929, Mather was the first general hospital in the Town of Brookhaven. It had 54 beds, a laboratory and an emergency room with a single bed, according to the hospital’s website.

A Northwell takeover of Mather would add to its expanding coverage in Suffolk County.

Four hospitals in the county have joined Northwell’s network in a little more than two decades: Huntington Hospital in 1994, Southside in Bay Shore in 1996, South Oaks in Amityville in 2012 and Peconic Bay Medical Center in 2016.

Northwell also opened its first cancer care center in Suffolk County last year. The $46.5 million, 46,000-square-foot Imbert Cancer Center replaced a former King Kullen supermarket at 440 E. Main St. in Bay Shore.

“Together, Mather and Northwell will play a crucial partnership role expanding world class care and innovative patient services to Suffolk County residents,” said Michael J. Dowling, Northwell’s president and chief executive, in a statement.

New Hyde Park-based Northwell, which is the largest private employer in the state, with 62,000 employees, operates 22 hospitals and more than 550 outpatient facilities, 95 of which, including 10 urgent care centers, are in Suffolk County. Eleven of the hospitals are on Long Island.