The agency landed the business after a two-way shoot-out against the incumbent on the above-the-line business, Delaney Lund Knox Warren & Partners.
Adrian Mack, the new media manager at Freeview, handled the pitch.
Grand Union will now work on Freeview's digital strategy, creative development, website and online advertising activity. The agency will work closely with DLKW, which scooped the advertising business in September following a pitch against St Luke's, CHI & Partners and Farm.
Grand Union has already worked with Freeview on a web project, which it won in September without a pitch.
Freeview, which provides free-to-air digital TV channels, radio stations and interactive services through an aerial, is currently the most popular way of viewing digital TV in the UK, with more than 14 million homes signed up.
Mack said: "We approached Grand Union for a one-off project and were very impressed with the team and their approach. On the back of this, we decided to offer them our wider web business and a place on the pitchlist for the advertising."