According to China's Manned Space Engineering Office, the Tiangong-1 "mostly" burnt up above the vast ocean's central region at 8:15 am Beijing time (0015 UT), on Monday.
Chinese authorities had said the spacecraft was unlikely to cause any damage when it comes down and there has been no immediate confirmation of any remaining debris.
Tiangong-1 or "Heavenly Palace" was placed in orbit in September 2011, acting as a testing ground for China's efforts to build its own space station by 2022, but ceased functioning in 2016.
Space officials had warned that knowing the exact location of the re-entry would not be possible until shortly before it happened.
However, the European Space Agency had described the probability of someone being hit by debris from Tiangong-1 as "10 million times smaller than the yearly chance of being hit by lightning".
The module, which was used to practice complicated manual and automatic docking techniques, was originally intended to be used for just two years, but ended up serving considerably longer.
It hosted Chinese astronauts on several occasions as they performed experiments and even taught a class that was broadcast into schools across the country.
Since 2003, China has become the third country capable of launching humans into space, following the former Soviet Union and the United States.