Two people, backed by an accomplice, burst into the museum of hunting and nature in Paris' historic Marais district at around 2pm, neutralised the guards and made off with the horn of a rhino captured in South Africa in the 1980s.
The security guards were briefly treated for the effects of the stun gas.
European museums, zoos and auction rooms are on alert following a spike in robberies involving rhino horns, fuelled by an illegal trade towards Asia and the Middle East where a horn can fetch tens of thousands of euros.
The rhinos' ivory is ground up and used in traditional medicines for fevers, convulsions and as an aphrodisiac.
This year alone in France, horns were stolen from western Rouen in March and from central Blois and the western Island of Aix in July, in addition to a botched robbery in central Bourges.
Thoiry zoo west of Paris has put its three white rhinos under surveillance to protect them from poachers.
Elsewhere in Europe, two horns were stolen in recent months in Vienna, from a taxidermist and an auction room. In Lisbon police arrested two Australians with six horns in their luggage, while in Britain thieves stole two horns from the natural history museum in southern Tring -- which were in fact copies.