The ring recruited adult women and girls between the ages of 14 and 17 in a school and in nightclubs in the southern region of Murcia, as well as over the Internet, to work in brothels, a ministry statement said.
Police arrested 29 people as part of the operation, including three women suspected of running the ring, and several of their clients who would specifically ask for under-age girls.
"The ring had put together a photobook which offered 'a la carte' over 400 women, including minors," the statement said.
Authorities did not say how many under-age girls were featured in the photobook but the interior ministry said it had identified 12 such victims of the ring.
Police were seeking further suspects with an interest in minors as well as more victims of sexual exploitation by the gang.
Police launched an investigation in September 2014 after a 17-year-old Ecuadoran girl went missing.
The girl's parents told police that before disappearing she had "a high standard of living", though she was not known to be working.
Police found the girl living in an apartment in Murcia with other youths, including several other underage girls, where they were being exploited by the owners.
The flat belonged to a prostitution ring run by three women, each of them operating a brothel apartment with its own prostitutes. The photobook allowed clients to order the services of prostitutes at any of the brothels.
The network ran ads on the Internet looking for women to work as bartenders, maids or to look after elderly people, and would then try convince those who replied to work as "masseuses".
In some cases they tricked the parents of the minors recruited by pretending to be mothers looking for someone to look after their children.
Prostitution is not illegal in Spain, but prostituting others for personal gain is against the law.