"They are mostly mothers who have come here to feed their families," Marianne Schweizer of the Swiss sex workers aid group Aspasie told Geneva daily Le Matin recently.
Figures from Geneva city police show that there were 80 Spanish women officially registered as prostitutes in 2010.
That number is now four times higher, with twice as many Spaniards thought to be working illegally, according to local authorities.
However, these sex workers described as Spanish may also number Romanians and Bulgarians in their ranks.
This is because women from those countries use Spain as a way station. Both Spain and Switzerland are part of Europe's Schengen zone which allows for the free movement of people, whereas the Eastern European countries are excluded.
The rise in the number of 'Spanish' sex workers in the city near the French border is also part of overall growth in the city's sex trade, with Le Matin.
There were 800 prostitutes registered in the city in 2004 but that number was 4,140 in 2011.
The competition has pushed prices down from around 100 Swiss Francs (€80, or $109) for a 15-minute encounter to as low as 20 Swiss Francs, one sex worker told Le Matin.