Joe Ledley's ninth-minute thunderbolt halted Barnsley's giantkilling run and secured Cardiff their first FA Cup final appearance since 1927.
The Wales international lashed home Barnsley defender Rob Kozluk's headed clearance from just inside the penalty area to maintain the Bluebirds' habit of scoring an early goal in the last three rounds.
Barnsley, who conquered both Liverpool and Chelsea in previous rounds, created several chances of their own and ran themselves into the ground in front of a record attendance FA Cup semi-final attendance of 82,752, with Kayode Odejayi missing the best chance in the second half.
Meanwhile, Manchester United's Premier League title push has run into trouble after they were held to a 2-2 at Middlesbrough.
Everton have kept up the pressure on fourth placed Liverpool with a 1-0 victory over Derby at Goodison Park.