Among the smallest frogs in the world, they live on the forest floor and make insect-like calls at night according to the report in the PeerJ medical sciences journal.
The researchers, who spent five years in the forests of Kerala and Tamil Nadu, said the tiny amphibians were there in abundance but had likely been overlooked because of their size.
The lush Western Ghats mountain range, which runs parallel to the western coast of India, is home to hundreds of threatened plants and animals.
"These tiny frogs can sit comfortably on a coin or a thumbnail," said Sonali Garg of the University of Delhi, who was among the team that found the new creatures.
"We were surprised to find that the miniature forms are in fact locally abundant and fairly common.
"They were probably overlooked by researchers because of their extremely small size, secretive habitats and insect-like calls."
The night frog group, Nyctibatrachus, previously had 28 recognized species, of which three were miniature in size (less than 18mm).
The new species were identified through studying their DNA, physical features and call patterns, the BBC reported.
Most of the new frogs live outside protected habitats or close to areas of human habitation.
Prof SD Biju of the University of Delhi, who led the research, has discovered over 80 new species of amphibians from India. "Over 32%, that is one-third of the Western Ghats frogs, are already threatened with extinction," he said.
"Out of the seven new species, five are facing considerable anthropogenic threats and require immediate conservation prioritization."
Indian night frogs split off from other frogs some 70 to 80 million years ago, making them a particularly ancient group.