Argos began the review of its advertising account in December 2013, when it appointed AAR to manage the pitch.
The brief was to "reinvent Argos as a digital retail leader" and was led by the marketing director, Stephen Vowles, who joined Argos in September last year.
Speaking about the reappointment, Vowles said: "We saw some really exciting ideas on how we can reposition Argos as a more universally appealing, digitally-led business, and we're looking forward to continuing our strong relationship with the CHI & Partners team to make these ideas a reality."
JWT London, Publicis London and Rainey Kelly Campbell Roalfe/Y&R also pitched for the account alongside CHI & Partners and Now but were dismissed before the final round.
Melissa Robertson, a managing partner at Now, said: "We were thrilled to get down to the last two on such a competitive pitch. Argos have been great clients to work with, but huge congratulations have to go to CHI for retaining the account."
CHI & Partners has worked with Argos since 2005, when the agency won a four-way pitch against DDB London, WCRS, and the incumbent Euro RSCG, which had held the business for three years.
Paul Phillips, the managing director of AAR, said: "Goldie [Sarah Golding, CEO] and the team should be congratulated for not simply retaining the business, but winning the pitch against tough competition."