The entire Champollion team will relocate to Instinctif's London office, with the agency being formally renamed as Instinctif Partners, a spokeswoman for Instinctif said.
The fee for the acquisition has not been disclosed.
The agency's CEO Richard Nichols said Champollion brought expertise that "complements the agency's public policy offering and bridges into its corporate practice".
He said: "Along with the organic growth we have consistently seen at Instinctif for the past decade, this is another step on our journey to becoming a world-leading international business communications consultancy."
Champollion CEO Simon Buckby and MD Estelle McCartney will join Instinctif as managing partners and will report to senior managing partner Warwick Smith.
Buckby said: "I am delighted that after 12 successful years Champollion is joining Instinctif Partners. We have worked together in the past and are certain that the fit will bring new and exciting business opportunities for us all."
Known as College Hill before rebranding in 2014, Instinctif specialises in reputation, influence and engagement and currently employs 450 people in 20 offices across Europe, the Middle East, Africa, Asia and North America.
The acquisition follows a similar move by Four Communications in January, in which the group announced it had acquired John Lehal's Insight Consulting Group (ICG), creating a 50-strong public affairs team.