PRWeek reported in November that the charity, which oversees scouting in the UK, was asking agencies to bid either for one or both of the two briefs, with each worth up to £5,000 per month.
The association, which is one of the longest-standing clients of Portland's corporate division, confirmed it would continue working with the agency.
A spokesman said of Portland: "We believe they can help us improve the reach and relevance of our comms work going forward to help even more young people get key skills that will help them in their future lives.
"We are delighted to have been appointed by The Scout Association for third time, taking our relationship into its second decade", said Portland partner Rebecca Gwilliam, who said that the agency would help the organisation "continue to communicate its relevance and increase its reach".
Separately, the Scout Association is currently preparing to launch a new brand, which will initially be activated internally from next month. Portland will assist with launching it externally later this year, although it did not work on its redevelopment.
Former PRCA Council chair David Hamilton, the Scouts' comms chief, was last month listed as one of the most powerful people in charity PR in PRWeek's Power Book.