PRWeek reported last week that 3 Monkeys Zeno failed to retain the consumer account and admitted it was "surprised" the pitch was called.
Agency MD Christine Jewell said at the time: "We wish them well with their new PR partner and look forward to using our skills and contacts to the advantage of other retailers."
Weber Shandwick's new remit includes corporate and consumer PR activities, and will be led by chairman Jon McLeod and MD Heather Blundell out of the agency's Manchester office.
McLeod said: "There are tremendous opportunities for growth within the business to challenge and disrupt the old and new incumbents in the sector, and we look forward to playing our part in that."
Iceland also works with sole operator Keith Hann, who handles corporate PR. Hann told PRWeek his relationship was unaffected by the appointment.
"Weber and I will be supporting each other in the same way as they will be working with Iceland’s in-house consumer PR team," Hann said.
Weber Shandwick, which is ranked in the PRWeek Top 150 2017 as the UK's third largest consultancy, previously enjoyed a long-standing relationship with budget supermarket Aldi, until being replaced by Red Consultancy earlier this year.