The 19-year-old has repeatedly been linked to the Reds in recent weeks and has signed a four-year contract with the Barclays Premier League club.
He will go straight into the Merseysiders' first-team squad, joining fellow summer arrivals Andrea Dossena, Diego Cavalieri and Philipp Degen.
Ngog, who is the cousin of former Rangers and Newcastle defender Jean-Alain Boumsong, struggled in an under-performing Paris St Germain team last season as the French club barely maintained their Ligue 1 status.
He scored just once in 18 appearances but has been far more effective at various levels for France.
The pacy forward has scored at every youth level for his country, including a brace against England Under-19s last season.
His exit from the Parc des Princes seemed inevitable from the moment he refused to sign a new contract with Paul le Guen's side as he entered the final year of his deal, and Liverpool soon stepped in with a bid thought to be around the Â£1.5million mark.