The agency won the brief following a competitive pitch against eight undisclosed agencies and its work with eBay will launch later this year, with a campaign geared towards the Christmas trading season.
Fetch claims that mobile already accounts for a considerable amount of eBay's business, and will play a significant role in its Christmas media strategy.
James Connelly, managing director for Fetch, said: "We have long considered eBay to be one of the most innovative brands in the mobile space, so, as an agency that’s passionate about innovation, we're extremely excited about working with them.
"We've been using eBay as a best-in-class case study for years, and we're looking forward to helping eBay strengthen its mobile offering, and engage with consumers across multiple platforms."
The eBay win follows a run of new business wins by the agency, including Square Enix and Bloom FM.
Earlier this year, Fetch hired Bigmouthmedia founder Steve Leach in the newly created role of chairman.
In September, eBay redesigned its logo for the first time in 17 years.