Trollope was previously first-team coach at Birmingham City and Norwich City, working alongside Chris Hughton.
During five years as manager of Bristol Rovers he guided the club to promotion to League One in 2007.
Trollope's arrival comes as Kevin Nicholson was made Under-21 manager following Quinton Fortune's departure.
The former Manchester United player had been in the role since July 2014 when he had been called in by former manager Ole Gunnar Solskjaer.
Swindon-born Trollope won nine caps for Wales between 1997 and 2003.
He was a guest in the Chairman's Suite for Cardiff City's 0-0 home draw with Brighton on Tuesday night.
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