"We are delighted to land a young player with such a fantastic future ahead of him," Portsmouths chief executive Peter Storrie told the clubs official website.
"There is no doubt other big clubs wanted him but as I always said when the rumours started flying we had an agreement with Haifa.
"It is tremendous news for the club and tremendous news for the supporters who have really taken to Yakubu in a big way.
"We have a real player for the future and I would like to echo the chairman's words in saying that it has been an absolute pleasure to do business with Maccabi Haifa."
Chairman Milan Mandaric told the club's official website, www.pompeyfc.co.uk, "I don't need to tell fans about the player's potential and we have a real jewel in the crown before the current season has even ended."