By citing a time frame in an address to the U.N. General Assembly, Netanyahu - who has clashed with President Barack Obama over the urgency of military action against Iran - appeared to suggest no Israeli attack was imminent before the November 6 U.S. presidential election.
Holding up a cartoon-like drawing of a bomb with a fuse, Netanyahu literally drew a red line just below a label reading "final stage" to a bomb, in which Iran was 90 percent along the path of having sufficient weapons-grade material.
Experts put that at the point that Iran has amassed enough uranium, purified to a level of 20 percent, that could quickly be enriched further and be used to produce an atomic bomb.
Netanyahu told the United Nations he believes that faced with a clear red line, Iran will back down in a crisis that has sent jitters across the region and in financial markets.
"And this will give more time for sanctions and diplomacy to convince Iran to dismantle its nuclear weapons program all together," he added.
Netanyahu never explicitly said that if Iran crosses his red line, Israel would launch attacks against the Iranian nuclear facilities, but he did seem to imply such a threat.
"At this late hour, there is only one way to peacefully prevent Iran from getting atomic bombs. That's by placing a clear red line on Iran's nuclear program," Netanyahu said.
Iran, Netanyahu said, was well into what he defined as the second stage of enrichment - 20 percent purification - and predicted it would complete it by "next spring, at most by next summer, at current enrichment rates."
According to an August report by the U.N. International Atomic Energy Agency (IAEA), Iran has stockpiled 91.4 kg (201.5 pounds) of the 20 percent material.
Some experts say Iran would need 200 to 250 kg (440 to 550 pounds) of such material for a weapon. Other experts suggest less might do it. Iran could potentially reach that threshold soon by producing roughly 15 kg (33 pounds) a month, a rate that could be speeded up if it activates new enrichment centrifuges.
According to the U.N. nuclear watchdog, around 25 kg (55.1 pounds) of uranium enriched to a 90 percent purity level would be needed for a single nuclear weapon.
In his own speech to the General Assembly on Tuesday, Obama said the United States will "do what we must" to prevent Iran from acquiring nuclear weapons and that time is not unlimited for diplomacy to resolve the issue.
Obama set no ultimatum or clear "red line" of his own, despite public urging from Netanyahu over the past several weeks that has aggravated strains between the two leaders.