Rumsfeld said Polish Defense Minister Jerzy Szmajdzinski told him about the find when they met earlier this week at a NATO summit in Istanbul.
"He pointed out that his troops in Iraq had recently come across -- I've forgotten the number, but something like 16 or 17 -- warheads that contained sarin and mustard gas," Rumsfeld told Newradio 600 KOGO of San Diego, California, in an interview aired Wednesday.
"Now these are weapons that we always knew Saddam Hussein had that he had not declared, and they have tested them," he said.
The Pentagon released a transcript of the interview on Thursday.
The head of the US-led hunt for weapons of mass destruction in Iraq, Charles Duelfer, had reported in a June 24 interview with Fox television that 10 to 12 rounds containing sarin or mustard gas had been found.
Pentagon spokesman Larry DiRita said Rumsfeld apparently was referring to be the same warheads that Duelfer had mentioned.
A US official, speaking on condition of anonymity, said the number could have gone up since Duelfer spoke about them.
Rumsfeld added that he had not seen the weapons or the results of the tests, but noted that the Poles believed they "in fact were undeclared chemical weapons -- sarin and mustard gas -- quite lethal."
"And that is a discovery that just occurred within the last period of days," he said.
Rumsfeld also said there had been "a lot of intelligence speculation and rumors and chatter about the fact that Saddam Hussein may have placed some of his weapons of mass destruction in Syria prior to the start of the war.
"Until that can be validated and proved, you'll find people in the administration not talking about it," he said.
The question of what happened to Iraq's weapons of mass destruction -- or indeed whether it had any hidden stockpiles at the time of the war -- remains intensely controversial.
David Kay, the former head of the weapons hunt, said on stepping down in February that it was unlikely there were any large stockpiles of weapons when US-led forces invaded Iraq last year, and no evidence had been found of even small stockpiles.
On May 2, however, US troops found a 1991 Gulf War-era mortar round with mustard gas that had been rigged to explode in a median of a road west of Baghdad.
Two weeks later, soldiers found a 155mm artillery round that tested positive for sarin gas, a deadly nerve agent. It, too, had been rigged as a roadside bomb.
Duelfer said that since then other sarin and mustard rounds have been found, some of them in southern Iraq in areas that were former weapons depots. They were made more than ten years ago, he said.
While they showed that Iraq's pre-war declarations to the United Nations were wrong, Duelfer said he could not say Iraq had hidden a "militarily significant" stockpile of chemical weapons.
Nevertheless, he expressed concern that anti-coalition insurgents such as Abu Mussab al-Zarqawi, were trying to tap into the expertise of former Iraqi weapons scientists.