The five-foot piece of metal, including a clearly visible Boeing identification number, was found wedged between two New York City buildings, police said.
It was found on Wednesday by surveyors inspecting a lower Manhattan building.
Nearly 3,000 people died in the terror attacks as planes were brought down in New York, Washington and Pennsylvania.
Five suspected al-Qaeda militants are awaiting trial for the attacks at a military tribunal at the US detention facility in Guantanamo Bay, Cuba.
2,976 counts of murder The New York Police Department has secured the area behind 51 Park Place and 50 Murray Street as a crime scene.
The location is at the site where a mosque and community centre has been proposed, three streets away from "Ground Zero" - the site of the twin towers.
Police have taken photographs and are keeping it off-limits until a health assessment has been made by the medical examiner's office.
After that, NYPD Deputy Commissioner Paul Browne said "a decision will be made concerning sifting the soil for possible human remains".
NYPD said the landing gear was found after surveyors hired by the property owner inspecting the rear of 51 Park Place called police on Wednesday.
Analysts suggest a full exploration of the site may require some demolition work on the two buildings.
At 08:46 on 11 September 2001, American Airlines flight 11 hit the World Trade Center's north tower. Seventeen minutes later, United Flight 175 hit the south tower.
Although rubble from the attack was cleared in 2002, other debris has been found scattered across the local areas in the years since.
Khalid Sheikh Mohammed is accused of masterminding the attacks while the other four men being held at Guantanamo Bay are implicated for providing support for the co-ordinated hijacking.
They are charged with conspiring with al-Qaeda, terrorism, and one count of murder for each known victim of the 11 September attacks at the time the charges were filed - 2,976 in total.
The five face a possible death penalty sentence if convicted.

bbc.com