Patrick Kiely of East London, pleaded guilty to the attempted theft of a rhino horn from Norwich Castle Museum in a failed robbery attempt committed on the 20th February 2012.
The horn is said to be worth in the region of £500,000.
Four men entered Norwich Castle and forced open a display case. Museum staff disturbed the men. The horn was recovered successfully.
The men, who were caught on CCTV, were later arrested.
The Museum has since replaced the rhino horn with a replica. Rhino horns can sell for about £50,000 per kilo on the black market.
Kiely is already serving a six year sentence over the theft of 18 Chinese jade items from Cambridge’s Fitzwilliam Museum.
Up to £15m worth of jade artifacts were taken in the theft on April 13.
Kiely has been sentenced to serve a further 18 months added to the end of his current six-year term.
