Robert W. Alexander, a third-generation firefighter who lived on Long Island as a child, died Monday of cancer linked to his work at Ground Zero — eight months after the death of his father who also worked on the pile.

A retired FDNY firefighter and NYPD officer, Alexander, of the Bronx, died Monday of brain cancer, his family and fire officials said. He was 43.

His father, FDNY Lt. Raymond W. Alexander, 76, died Nov. 21, making the Alexanders the department’s first father-and-son duo to die from 9/11-related illnesses, officials said.

FDNY Commissioner Daniel Nigro paid tribute Thursday to the Alexander family and its long history of service.

“World Trade Center illnesses continue to take the lives of far too many FDNY members, now including two generations of the Alexander family — a father and son who served so bravely, for so long,” Nigro said in a statement.

Robert Alexander was an NYPD officer on Sept. 11, 2001, when terrorists struck the Twin Towers, the FDNY said. He wasn’t working that day, but was called in to assist and worked at Ground Zero for weeks, the family said.

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His father was a 40-year FDNY veteran, and Alexander’s grandfather was also a firefighter.

Alexander joined the FDNY in 2002 and was assigned to Ladder Co. 45 in Harlem. He later served on a Hudson River fireboat before retiring as a chief marine engineer in 2015, the FDNY said.

“He was very proud of his heritage — that he came from a family of firefighters,” said a cousin, Lori J. LaPonte, of Mount Sinai.

Born in Syosset, Alexander attended elementary school in East Northport. The family moved to western Massachusetts when he was about 10. He graduated in 1995 from SUNY Alfred and worked as a diesel mechanic.

After working as an EMT in the Bronx, Alexander became a city police officer, with assignments in Queens and Manhattan.

In 2014, he began having headaches and was diagnosed with a brain tumor, the family said.

Gerard Fitzgerald, president of the Uniformed Firefighters Association, the union representing the FDNY, said Alexander was the 11th firefighter to die from a 9/11-related illness this year.

“Bobby had a constant presence in Washington, D.C. during the reauthorization of Zadroga Act,” Fitzgerald said. “Even while battling his cancer, he would show up to rallies and events to highlight his sufferings.”

The funeral will be held Saturday at Our Lady of the Valley Roman Catholic Church in Sheffield, Massachusetts.