Dozens of foreign writers, including Turkey's Pamuk -- a free speech campaigner once charged with "insulting Turkishness" -- are due in Sri Lanka for the festival in the southern town of Galle from January 26.
"RSF finds it highly disturbing that literature is being celebrated in this manner in a land where cartoonists, journalists, writers and dissident voices are so often victimised by the current government," the group said.
"We believe this is not the right time for prominent international writers like you to give legitimacy to the Sri Lankan government’s suppression of free speech," it told the attending authors.
The RSF launched the campaign with Sri Lanka's Journalists for Democracy (JDS).
They have been backed by authors Noam Chomsky, Arundhati Roy, Ken Loach, Antony Loe