The head from the costume that tops Bernie's outfit was last used for the Northampton Saints v Saracens game at Franklin's Gardens on Saturday, February 8.
It was last seen in the DJ roadshow unit in the Franklin's Gardens Lakeside Village just hours before the England A v France A game on Friday, February 14.
The head's absence was realised earlier today.
Northampton Saints communications manager Caroline Moore said: "Bernie is an integral part of a match day here at Franklin's Gardens, and special occasions, such as this weekend's Powergen Cup semi-final against London Irish.
"Bernie is also crucial to the match day enjoyment of our young fans, many of whom are members of Bernie's Gang. The children will certainly miss his presence this weekend if Bernie's head is not returned or found in the next few days.
"Although it is amusing, it is also annoying. The head of the Bernie costume is worth £2,000 on its own and we will struggle to get one made before Sunday. Everyone at the club, including the playing squad, is disappointed that Bernie's head has gone missing at such a crucial stage in the season."
Bernie was present at the Tetley's Bitter Cup and Heineken Cup finals in 2000, as well as last year's Powergen Cup final. He is present at every home game and does numerous charity visits, including two run outs in the Mascots Grand National at Huntingdon and last year's Flora London Marathon.
Everyone at the club would appeal to anyone who knows anything about the disappearance of Bernie's head to call Franklin's Gardens on 01604 751543.