San Suu Kyi is charged with illegally importing and using communication equipment, according to the Kyodo news agency.
Her detention was due to expire on Monday.
The Myanmar democracy icon was detained along with other top officials on 1 February, when the military seized power after accusing her party of rigging the November elections. Most of those detained have since been released, but San Suu Kyi, ousted President Win Myint and other party leaders were placed under house arrest.
The military claims its commitment to democracy and vows to hold new and fair elections after a year-long state of emergency ends.