Bell Pottinger chief executive James Henderson told PRWeek the agency has started the search to replace Lotinga, who together with PA chairman Tim Collins heads a team of 35.
Edelman has a gap in its public affairs offering due to the resignation of its MD Chris Rumfitt in November, though it is not clear at this stage if Lotinga is a direct replacement.
Bell Pottinger is losing a well-connected Liberal Democrat who it hired from the party as an account director following the 2010 general election.
Lotinga was a senior adviser to the party on public services and health before the change of role.
"Steve has done a great job for us and we’re disappointed to see him go," Henderson said.
Edelman general manager Alex Bigg said: "In Stephen we feel we have found someone who is not only an exceptional political consultant but understands the changing communications landscape."
Rumfitt, who is setting up his own agency, was due to leave Edelman at the end of this month.
Lotinga declined to comment.