Three UK has picked Zenith to work on its media planning and buying, ending its 13-year relationship with Mindshare.
The mobile network called a statutory review earlier this year, around the same time that it appointed MRM McCann as its customer experience agency.
Zenith will begin working on the business in the first quarter of 2020. Three also works with Wieden & Kennedy London on above-the-line advertising and Gravity Road on social media.
Shadi Halliwell, chief marketing officer at Three, said: "The way people consume media is changing and our brand continues to evolve and grow. Our aim was to find an agency partner who truly understands how customers engage with media. Zenith demonstrated a solid understanding of our needs through an innovative and disruptive approach.
"2020 presents a huge opportunity for us to pave the way with our superior 5G offering within the connectivity market. Like all our agency partners, they will become an extension of the Three marketing team, joining us as we move into a leadership position through our 5G offering."