A former Brighton trainee, Piercy has impressed Seagulls boss Martin Hinshelwood with his displays in training and with Brighton's reserves during his four-week stay and will be included in the squad that travels to Rotherham United on Saturday.
On his new signing, Hinshelwood told the club's official website: "He's been here for four weeks and we've been very impressed with him.
"He's a player who can play in a variety of positions. He can play wide or in the middle - when he made his debut for Tottenham in The Premiership he played up front.
"He's the right age and his ability to play in a number of different positions will give us plenty of options."
The 22-year-old was close to signing for the Withdean Stadium club last March but it fell through after he picked up a groin injury in a Spurs reserve match.
Piercy decided not to sign a new deal with the Premiership high-flyers in order to find more first team action with another club.
He said: "They (Tottenham) offered me a new deal there this season, but I wanted to go somewhere where I would be more involved and would feel happier.
"I live in Eastbourne, and I know Dean White well and he said come down and have a trial, and it's worked out really well.
"What I enjoy most is attacking and running at people, if I can do that then hopefully everyone will be happy."
Brighton will hope Piercy can boost the fortunes of last season's Division Two champions, who have lost their last seven league matches and sit second from bottom in Division One.
