The drugs were found after the truck was pulled over for a secondary inspection and drug-sniffing canines picked up an odor, Michael Humphries, the Nogales Area Port director, told reporters.
Another 395 pounds (180 kg) of methamphetamine, worth about $1.1 million, also was confiscated from the truck, officials said.
The load was enough for more than 100 million lethal doses of the drug, a synthetic opioid that has fueled an epidemic of US overdose deaths.
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U.S. Customs and Border Protection officials in Arizona say they've made their biggest fentanyl bust ever.
The nature of this seizure runs counter to what President Trump has said about the way in which drugs enter the USA from Mexico. "Thanks, as always, for a job well done!"
The drugs were seized from a Mexican national crossing through the port of entry in Nogales.
"It was totally a cold hit" based on the agent's hunch, Coleman said.
Fentanyl has become the biggest source of fatal overdoses in the U.S.in recent years, the AP said, citing authorities.
Fentanyl was involved in almost 29% of all overdose deaths in 2016, the government report said. The AP added that it's seen as a white powder that's mixed with heroin for an extra kick and in the form of blue pills that are counterfeits of prescription drugs like oxycodone.
The federal Centers for Disease Control and Prevention said in a recent report that fentanyl is now the drug most often involved in fatal overdoses across the country, accounting for more than 18,000, or nearly 29 percent, of the 63,000 overdose fatalities in 2016.