He said he conveyed the concerns personally to Chinese officials during his visit to Beijing this week.
"Just the other day, Monday, when I was in Beijing, this was one of the issues I raised — concern about Chinese weapons or Chinese-designed weapons showing up in some of these battle areas, be it Iraq or Afghanistan," he told a congressional hearing on US-China ties.
The US military in Iraq had said its troops had found Chinese-made missiles which they believe were smuggled in by groups in Iran aiming to arm militants fighting US-led forces.
The US military has repeatedly accused Iranian-linked groups of training Iraqi extremists in the use of armor-piercing weapons known as explosively-formed penetrators (EFPs).
Afghan authorities had also seized dozens of Iranian and Chinese-made weapons from Taliban fighters near the border with Iran.
The weapons, including land mines and rocket-propelled grenades, appeared to have been manufactured in Iran and China, reports have said.
Negroponte told the US senators that Washington "made it clear" to Beijing that Chinese entities' continued sale of conventional weapons to Iran "is unacceptable."
He said Chinese officials told him "they have scaled way back their sale of conventional weapons to Iran."
"They had relationships previously where they exported these weapons but they have dialed that back," he said.
Negroponte, a former US intelligence czar, said that China understood Washington's position that Iran presented "a grave international and regional security concern."
Beijing also understood that the US government "reserves the right to apply all multilateral, bilateral and unilateral measures at our disposal to ensure that our concerns are addressed."
"We reinforce this message at every opportunity," he said.
Negroponte also slammed China for expanding trade and investment links with Iran, particularly in its oil and gas sector.
"We believe this expansion undermines international efforts to pressure Iran, and sends the wrong signal to the Iranian regime, especially at a time when other oil companies are heeding their governments' wishes to forgo new investments in Iran," he said.
Negroponte said the Chinese government shared America's "strategic objective of preventing Iran from acquiring a nuclear weapons capability."
Washington has spearheaded efforts at the United Nations to rein in Iran's ambitions to master the nuclear fuel cycle, accusing Tehran of seeking to build atomic weapons.
China has voted in the UN Security Council for sanctions on Iranian individuals and companies linked to nuclear weapons and ballistic missile programs.
