The statement posted on Ayatollah Ali Khamenei's website marked his first reaction to the idea of holding the talks, a proposal that had been floated for months and was reinforced last week by US Vice President Joe Biden.
Momentum depended on Mr Khamenei, who has the final say in all key matters and has previously opposed direct negotiations.
In a statement, he said: "Talks will not solve any problems."
In an apparent reference to US sanctions over Iran's nuclear efforts, he added: "You are holding a gun against Iran saying you want to talk.
"The Iranian nation will not be frightened by the threats."
Iran and six world powers, including the US, are scheduled to resume nuclear negotiations later this month.
Talks last year ended in stalemate, with Tehran pushing for a rollback in sanctions in exchange for any key concessions on its programme.
The West and its allies fear Iran's uranium enrichment labs may eventually produce weapons-grade material.
The Islamic Republic says it only seeks nuclear fuel for energy reactors and medical applications.
The US and Iran broke ties after the storming of the American embassy in Tehran in the wake of the 1979 Islamic Revolution.