The company makes products including the iPhone, Dell computers and Nokia phones.
Police said Li Hai, 19, killed himself after working at the plant for only 42 days, the official Xinhua News Agency reported.
Foxconn did not immediately comment on the death.
It was the 10th suicide this year at the world's largest contract maker of electronics, which also makes Dell computers and Nokia phones.
The suicide is the ninth at Foxconn's massive plant in the southern city of Shenzhen, which employs more than 300,000 people.
Two other workers have tried to kill themselves by jumping from buildings in Shenzhen but they survived.
Another suicide occurred at a smaller plant in northern Hebei province in January.
Labour activists say the string of suicides back up their long-standing allegations that workers toil in terrible conditions at Foxconn.
They claim shifts are long, the assembly line moves too fast and managers enforce military-style discipline on the work force.
In Hong Kong, about a dozen labour activists protested at Foxconn offices in the Chinese territory.
They held signs that said, "Foxconn lacks a conscience" and "Suicide is no accident."
The protesters, from the Hong Kong Confederation of Trade Unions, burned cardboard cutouts resembling iPhones.
But Foxconn has insisted that workers are treated well and are protected by social responsibility programs that ensure their welfare.
The Shenzhen factory is perennially a popular place to work, with hordes of applicants lining up for jobs during the hiring season.
Earlier this week, Foxconn chairman Terry Gou said: "We are certainly not running a sweatshop. We are confident we'll be able to stabilise the situation soon."
Yesterday's reported death came just three days after a 21-year-old man who worked in the logistics department jumped from a four-storey building shortly after finishing the night shift.
The highest-profile Foxconn death happened last July, when Sun Danyong (25) jumped to his death after being interrogated over a missing iPhone prototype.
