CHI & Partners already handles above-the-line advertising for TalkTalk as well as The Carphone Warehouse.
Buying on the account will be supported by Group M, the media buying point and part of the WPP group, which took a 49% stake in CHI last year.
The account was previously handled by incumbent Essence, which will be retained for ongoing online project work.
The move brings all of The Carphone Warehouse's media planning and buying across the group under CHI & Partners Media.
CHI's media proposition offers clients a "one-stop shop" on a fee-only basis, rather than the traditional media agency commission model.
Ady Guthrie, e-commerce director at TalkTalk, said: "As a group we have decided to bring all our media planning and buying under one roof to take advantage of the economies of scale that this provides.
"We are excited to be working with Group M and taking advantage of the breadth and depth of their online direct response experience to drive greater volumes on broadband sales."
TalkTalk is the fourth planning and buying account win in the last year for CHI & Partners, which is headed by Tim Allnutt and Enyi Nwosu. It follows the capture of Virgin Money, Carphone Warehouse and Tiger Beer.
CHI & Partners Media currently heads Campaign's new business rankings for media accounts, with £43.3m worth of new billings since the start of the year.