Rossiter will join S4C in the autumn in the newly created post of director of communications. He will oversee marketing, press, internal communications and the web, reporting to S4C chief executive Iona Jones.
He will also be responsible for comms strategy as S4C prepares for full digital switchover in Wales from 2009.
‘The effective promotion of the S4C brand will be a priority for us in the multichannel era,' said Jones. ‘Huw's experience will add further expertise to S4C's management team.'
S4C last week claimed that it would lose ‘significant amounts of money' when it stops broadcasting English-language programmes, to focus on Welsh output, in 2009.
Rossiter joined Ofcom in September 2003 as it prepared for its launch. He led media relations around Ofcom's statutory review of public-service TV broadcasting (which S4C falls into).
He also handled consultations on TV food advertising to children.
Prior to Ofcom, Rossiter was head of public affairs at the Independent Television Commission.
Between 1992 and 2001 he was head of press and publicity at Cardiff-based BBC Wales. The previous ten years were spent in London as a BBC publicity officer.
Ofcom director of comms Matt Peacock said Rossiter's post will be reviewed in August.