Almost three weeks after she was reported missing, a young teacher was spotted floating in New York’s Upper Bay on Tuesday and was rescued by deckhands from a Staten Island ferry.

The teacher, Hannah Upp, 23, was plucked from the swells, taken ashore and transported to Richmond University Medical Center on Staten Island, where officials said she was in stable condition.

The disappearance of Ms. Upp, a New York City teaching fellow set to begin her second year as a Spanish instructor at Thurgood Marshall Middle School in Harlem, baffled her relatives and friends and touched off a citywide search. The teachers’ union offered a $10,000 reward for information.

After she vanished  on Aug. 29, just before the start of the school year  investigators found her keys, credit cards, wallet, cellphone and passport among her other belongings in her apartment in the Hamilton Heights neighborhood of Manhattan. She is believed to have last withdrawn money from her bank account on Aug. 28.