"Although we could have taken steps on this path, based on Islamic ruling we firmly and bravely said we won't take this path," Khamenei said.
"Both building and stockpiling it is wrong, as using it is haram," he told a group of academics in a video posted on his office's Twitter account.
"If we had a nuclear weapon, it would have been obvious that it would have been impossible for us to use anywhere," said the Iranian leader.
"Under Islamic principles it is definitely haram."
Iran vehemently denies having ever sought an atomic bomb and says its nuclear programme is for peaceful energy production and medical purposes only.
In May last year, President Donald Trump unilaterally withdrew the United States from a landmark 2015 deal that gave Iran relief from sanctions in return for curbs on its nuclear programme.