Freeney, a free agent after being released by the Indianapolis Colts in February, will help fill the gap left by outside linebacker Melvin Ingram, who tore the anterior cruciate ligament in his left knee during team in practice on Tuesday.
Though the Chargers did not reveal financial details, local media estimate Freeney's deal to be worth £8.8 million over the two years.
A first-round pick by Indianapolis in the 2002 NFL Draft, Freeney piled up numerous accolades while recording a franchise-record 107.5 sacks in 11 seasons for the Colts.
Freeney, 33, was a Pro Bowl selection in 2003, 2004, 2005, 2008, 2009, 2010 and 2011.
He spent the first decade of his career as a defensive end before switching to outside linebacker in 2012 when the Colts moved from a 4-3 defensive front to a 3-4.