The Japanese-Brazilian woman, 29, turned herself in to the Japanese consulate-general in Shanghai and was detained by local police for a suspected immigration offence, the Jiji and Kyodo news agencies quoted Japanese police sources as saying.
Japanese police are expected to seek her extradition, the reports said.
The body of nurse Rika Okada, also 29, was found in a storage lock-up in Tokyo. Investigators also found the two-metre box in which it had been transported from Osaka, 400 kilometres to the southwest.
The delivery service which ferried the package -- marked with the Japanese word for "doll" -- had been paid in Okada's own name, earlier media reports said. The bill for the lock-up's short term rental had been settled using her credit card.
The reports said the Japanese-Brazilian, who had been at elementary school with Okada, had flown out of Tokyo earlier this month using the dead woman's passport.
The schoolmate, who was not named, is believed to have lived just a few hundred yards from the lock-up with a Chinese woman of about the same age.
Both women flew from Tokyo's Haneda airport on the same flight to Shanghai, reports said.
Just before she went missing, Okada wrote on her Facebook page that she was going to meet an old friend whom she had not seen for a decade.