The 24-year-old joined Saints from Young Boys in August 2012 but made only 16 league appearances, spending the 2013-14 season on loan at Lorient.
On Monday night Metz, who was relegated from Ligue 1 in May, confirmed the arrival of 56-time Zambia international Mayuka at the Stade Saint-Symphorien.
Southampton boss Ronald Koeman had earlier this month revealed Mayuka was close to joining Russian side Krylya Sovetov, but the transfer failed to materialise.
