It's not one of the original Nobel Prizes but it is considered by the foundation to be on the same standing as the other prizes. Winners are selected by a committee appointed by the Royal Swedish Academy of Sciences but voted on by the Academy as a whole.
"In 1968, Sveriges Riksbank (Sweden's central bank) established the Prize in Economic Sciences in Memory of Alfred Nobel, founder of the Nobel Prize. The Prize is based on a donation received by the Nobel Foundation in 1968 from Sveriges Riksbank on the occasion of the Bank's 300th anniversary. The first Prize in Economic Sciences was awarded to Ragnar Frisch and Jan Tinbergen in 1969.
The Prize in Economic Sciences is awarded by the Royal Swedish Academy of Sciences, Stockholm, Sweden, according to the same principles as for the Nobel Prizes that have been awarded since 1901."
