Researchers say the discovery makes the island, which covers more than 65,000 square kilometres, an amphibian hotspot of global significance.
A team of US, Belgian and Sri Lankan scientists recorded the new frogs as they surveyed the island's disappearing wildlife.
Writing in the journal Science, the researchers led by Madhava Meegaskumbura, from the University of Boston in Massachusetts, US, called for the preservation of Sri Lanka's remaining forest fragments as well as habitat restoration.
Striking finds
They found that many species known from 19th Century museum collections had seemed to have vanished.
The frogs discovered in the census fit into two groups. One, comprising just five species, lay their eggs in foam nests in leaves, rocks or braches just above the water.
But most of the newfound frogs are "direct developers", which hatch as tiny adults, missing out the tadpole phase.
Discussing the disappearance of old, known species, Madhava Meegaskumbura said: "Given that the island has lost more than 95% of its rainforest habitat, this is hardly unexpected.
"Still, the persistence of so many species is striking, and may be attributable to a combination of terrestrial eggs, direct-developing embryos, and high fecundity."