The Cardiff coach will work as Chris Coleman’s national team coach, with Osian Roberts stepping up to replace Symons, who left his role as assistant boss last month to concentrate on the manager’s job at Fulham.
Trollope, who played alongside Coleman and Symons at Fulham and in the Welsh national team, will continue as Russell Slade’s assistant at Cardiff.
Coleman said: “On both a personal and professional level, we are very sad to lose Kit but I fully understand the reasons behind his decision and wish him the very best of luck at Fulham.
“I am however, delighted to welcome on board another former team-mate of mine, Paul Trollope, who is a young and highly thought of coach. Paul will be a great team member and will fit in perfectly with the staff.”
Trollope joins a Wales side that stands 10th in the FIFA rankings and in a strong position at the top of their Euro 2016 qualifying group.
Cardiff chief executive Ken Choo said: “As a club, we are proud and pleased to see Paul recognised by the FAW.
“Whilst his primary focus remains on the task at hand here at Cardiff City, we are happy for the national side to use Paul’s talent to their benefit, at agreed junctures, as they pursue a place in the European Championships.”