A spokesman for Indonesia's Geophysical Agency issued the warning after the quake hit on Friday, RT reported.
It struck 280 km (174 miles) south of the province of Gorontalo at a depth of 43 km (27 miles), according to the United States Geological Survey.
The warning was issued for coastal communities in the Morowali district where residents have been advised to evacuate. There were no immediate reports of damage or casualties, but the USGS warned that considerable damage was possible in poorly built or badly designed structures.
Indonesia lies in the so-called Pacific Ring of Fire, a vast area in the Pacific Ocean where many earthquakes and volcanic eruptions occur.
A 7.5-magnitude quake hit the other side of Sulawesi last year, triggering a tsunami which hit the city of Palu and killed more than 4,300 people.