A 9/11 hero fireman who rushed to Ground Zero on his day off -- and developed a irreversible lung injury after working in the toxic pit -- yesterday left a Manhattan hospital after receiving a lifesaving transplant last month.

With his 7-year-old daughter clinging to him and bagpipes playing "America the Beautiful," Lt. Martin Fullam -- who got a single new lung -- was wheeled out of Columbia Presbyterian Hospital as fellow firefighters, hospital workers and relatives cheered him on.

"I just want to say thank you to everybody -- this is not a road you take on your own," the retired firefighter said through a mask protecting him from infection. "I truly am the luckiest man in the world."

Fullam, 53, was diagnosed with polymyositis in 2005, one of six first responders who developed the autoimmune disease.

His wife, Patricia, noted that the "dark cloud" that descended when her husband fell ill now "is filled with a lot of silver linings."

His brother, fire Lt. David Fullman, said, "It's tough seeing your older brother go through this.

"Before, I could hardly hold a conversation with him . . . Having to push him around in a wheelchair is very difficult."

Yet the younger Fullman said his hero brother has not a single regret about taking a ferry from his Staten Island home to the fallen towers on 9/11.

"Marty said he would do it again and so would I," the younger Fullman said.

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