HH's music and television team, led by Shelley Botbol, will handle the account, promoting T4 presenters Steve Jones, June Sarpong, Vernon Kay and Miquita Oliver.
The brief also includes publicity around T4 flagship strand Popworld and events such as the Smash Hits Poll Winners' Party on 20 November and the NME Awards in February. It will also promote one-off programmes such as the Will Young T4 Special, due to air this Sunday.
C4 group publicity manager for entertainment Loretta de Souza said: 'T4 is an important brand for us and we wanted to give our publicity a boost.'
HH will target music and TV titles, women's magazines and national press. C4 previously used HH to promote chat show The Priory in 1999, The Big Breakfast in 2000 and the second series of Big Brother in 2001.