HardHat, which was founded in 2006 by former NHBC head of comms Andrew Howard, will be incorporated into the group's public consultation and stakeholder division Remarkable Engagement, which primarily operates in the housing, infrastructure, energy and retail sectors.
Howard, who was also formerly a local councillor for the Labour Party, will remain "heavily involved" in the business, a Remarkable Group spokesperson told PRWeek.
The addition of HardHat's staff will boost Remarkable's overall numbers beyond 90, with employees spread across its offices in London, Manchester, Cardiff, Leeds, Bristol, Norwich and Winchester.
According to Remarkable, the acquisition will add "substantial" value to both agencies' clients by delivering "more effective stakeholder and political engagement strategies".
"This deal delivers on a long-held ambition to set a new standard for the public consultation and stakeholder communications industry in the UK," said Remarkable Group chief executive Stephen Pomeroy, who also praised Howard's "powerful reputation" across the property, development, energy and infrastructure sectors.
Remarkable Group, which also acquired Pendragon Public Relations in 2014, is the UK's 58th largest PR firm by revenue, according to the PRWeek Top 150 consultancies 2016. Group revenue was £4.2m in 2015.
The acquisition follows a period of M&A activity in the sector in recent weeks.
Yesterday, US-headquartered global professional services firm Accenture confirmed it had bought Karmarama, the owner of PR firm Kaper, for an undisclosed fee.
Marcoms group Creston announced earlier this month that it had agreed to sell outright to investment firm DBAY Advisors, a current minority shareholder, for £75.8m, in a move that will end the group's period as a listed company. In October, London-based comms agency W acquired House PR.