The south coast club, who were relegated from the Premier League on Saturday and were rated 9-1 by bookmakers to beat Spurs, produced a battling display to reach the finale for the second time in three seasons having beaten Cardiff in 2008.
Tottenham had been expected to set up a May 15 showdown with Chelsea but after 90 minutes of stalemate, goals from Frederic Piquionne and former Tottenham player Kevin-Prince Boateng sent Portsmouth's long-suffering fans into delirium.
For manager Avram Grant, whose short reign at Pompey has been hampered by the cash crisis, transfer embargoes, points deductions and injuries to key players, it was a rare reason to smile as the club showed that it will go down with all guns blazing.
"We had the belief, we had the courage to come to this game to win this game," Grant, who will now face Chelsea, the club that sacked him after he took them to the 2008 Champions League final, on May 15.
"It's nice... blues against blues. It's unbelievable. These fans this year I will not forget in all my life. This belongs to the fans and the players. They deserve it. To see the fans this happy you work all your life for this."
NEWS:Grant revels in FA Cup limelight
Harry Redknapp, who steered Portsmouth to their FA Cup final triumph in 2008, their first since 1939, before leaving to transform Tottenham's fortunes, warmly congratulated Grant but was left cursing the Wembley pitch.
As in Saturday's semi-final between Chelsea and Aston Villa players often slipped on the turf - most tellingly in the 99th minute when Spurs skipper Michael Dawson lost his footing, allowing Piquionne to fire the opener.
"The pitch is a disgrace," Redknapp told reporters. "I'm not using it as an excuse but for any professional team to have to play on that is farcical. Michael Dawson slipped on it, it can't be right, can it?"
NEWS:Redknapp and Grant slam Wembley surface
Tottenham also had reason to feel hard done by when Peter Crouch had a goal disallowed shortly after Piquionne's opener but Redknapp was full of praise for his old club.
"People said before the match that we had got an easy game, but I knew it was never going to be an easy game. Full credit to Portsmouth. It's their day."
While Redknapp begins the post-mortem and tries to rally his side for Wednesday's crucial home Premier League match against Arsenal as they try and stay in touch for fourth place, Portsmouth at least have a silver lining to their torrid season.
RELEGATION TURMOIL
Of Portsmouth's starters on Sunday, only keeper David James began the Cup final in 2008, such has been the turmoil at the club which went into administration in February with debts of around 70 million pounds, triggering a nine-point deduction that all but condemned them to relegation.
Many of their players are unlikely to be at the club next season but for 120 m
