Francis told delegates to a drug-enforcement conference in Rome that even limited attempts to legalise recreational drugs "are not only highly questionable from a legislative standpoint, but they fail to produce the desired effects."
Likewise, providing addicts with drugs doesn't solve the problem and is "rather a veiled means of surrendering to the phenomenon," he said.
"Let me state this in the clearest terms possible: the problem of drug use is not solved with drugs!"
Just last month, Uruguay - next door to Francis' native Argentina - approved selling marijuana cigarettes in pharmacies.
Recreational marijuana is now legal in the US states of Colorado and Washington, and Oregon may put the matter to a vote later this year.
Marijuana is technically illegal in the Netherlands, but possession of small amounts is not prosecuted and it is sold openly in "coffee shops.