Trinity decided to put the four free newspapers in Northamptonshire and South Leicestershire up for sale earlier this year as part of its wide-ranging shake-up of its Midlands operations.
The titles concerned are: the Northampton Herald & Post, the East Northants Herald & Post Series, the Brackley & Towcester Herald & Post and the Market Harborough Herald & Post.
Iliffe chief executive David Fordham said: “Despite the current challenging market conditions, we firmly believe in the long term future of local media and by acquiring these titles we will significantly enhance our publishing business.
“We are delighted that, together with Trinity Mirror, we have reached an agreement which ensures these titles continue to serve their local communities.”
The four titles have a combined distribution of 113,000.
Earlier this month Iliffe announced plans to reduce staffing levels at two of its other subsidiaries, Staffordshire Newspapers and Cambridge Newspapers.