Christopher Schindler scored the decisive penalty in a 4-3 shoot-out victory after semi-final hero Danny Ward, on loan from Liverpool, kept out spot-kicks from Liam Moore and Jordan Obita.
That followed a goalless draw, with Reading winger Garath McCleary firing wide and Nakhi Wells spurning a great chance for the Terriers in extra-time as they couldn't be separated.
In the regular 90 minutes, Chelsea loanee Izzy Brown missed Huddersfield's best chance from two yards out while John Swift went close for the Royals.
The shoot-out win means Huddersfield will play in the Premier League for the first time, having last played in the top flight in 1972. They join Newcastle United and fellow PL debutants Brighton & Hove Albion in the PL next season.