The epicentre of the quake, which struck at 11:51 am (0451 GMT) at a depth of 25 kilometres, was 190 kilometres northwest of the town of Bengkulu.
Indonesia's Meteorology, Climatology and Geophysics Agency measured the magnitude at 6.1.
"There are no reports of damage so far. The quake happened in the sea close to Mentawai island, but no tsunami warning has been issued," the agency's duty officer, Koko Widyatmoko, told AFP.
Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.