Geneva prosecutor Jean-Bernard Schmid told Swiss newspaper Tribune de Geneve that "France has requested judicial assistance, and I accepted the various demands which were made by judge Renaud van Ruymbeke", one of the French judges involved in the investigation.
It is alleged that a 1995 presidential campaign by Sarkozy mentor Edouard Balladur was funded through a Pakistani submarine contract, which magistrates suspect indirectly led to a Karachi bomb attack that left 11 Frenchmen dead.
Schmid said Switzerland's assistance would only deal with the financial aspects of the case, adding that "due to current events, we need to work quickly."
Controversy over the arms contract erupted when investigators began probing whether a 2002 bomb attack in Karachi that killed 11 French engineers working on the project was a revenge attack for promised bribes not paid.
France's Constitutional Court was legally advised in 1995 that Balladur's campaign accounts should be rejected because of question marks over huge cash donations, but members eventually voted to approve them in a close vote.
Sarkozy came to office in 1997 vowing to lead an "irreproachable republic", but his camp has since been tainted by a series of scandals, and opinion polls show him likely to be beaten next year by a Socialist candidate.