Archant first acquired a minority (42.7%) stake in the business, which publishes Kent on Sunday and a series of free weekly newspapers and magazines, in 2005.
The two companies already had a strong relationship; as well as the minority stake, Archant also printed for KOS.
Archant chief executive Adrian Jeakings said: "We are delighted to bring KOS Media fully into our portfolio of regional titles. This acquisition strengthens Archant’s existing presence in Kent and provides a springboard for further investment."
The Norwich-based publisher, one of the six biggest publishers in the UK, appears to be strengthening its hold on a number of areas. Earlier this month, it launched an "alternative route to market" newspaper in Cambridge.
KOS Media Publishing recorded a £4.8m turnover to March 2009, its last recorded results. Archant Group had a £148m turnover in 2009.
Managing director of KOS, Paul Stannard, added: "I am delighted Archant has taken on full ownership of KOS Media. We will benefit from ownership by a large, well-financed, independent group with ambitious plans to grow the business."
