Officials said the group were hiking through Theni district, a hilly region in southern India covered in thick forest and dotted with tea plantations, when they were forced to flee a fast-approaching fire.
They reportedly became separated while trying to find a safe way to escape the oncoming blaze.
"It seems that the victims deviated from the regular trail. This is a popular trekking spot," Theni district official S. Kandasamy said.
"Nine people lost their lives and 27 others were rescued."
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Of those rescued, 18 were injured, Mr Kandasamy said. Some of the victims were taken to hospital with serious burns over most of their bodies, the Press Trust of India reported.
Mr Kandasamy said the fire is still burning uncontrollably in the region bordering Kerala state some 540 kilometres (335 miles) from the Tamil Nadu capital Chennai.
The cause of the blaze is being investigated, he added.
Helicopters from the Indian army have been enlisted to help douse the flames, PTI reported.
Emergency teams had accounted for all those on the hiking expedition, Theni district police superintendent V. Baskaran told AFP.
