A container with hidden packages of the drug was intercepted in the Pacific port of Lázaro Cárdenas, President Andrés Manuel López Obrador said.
"We already have proof," he said, adding that he would ask the Chinese government to help stop the shipments.
US authorities say fentanyl is now the main driver of US drug overdose deaths.
In March President López Obrador said he had written to Chinese President Xi Jinping requesting Chinese help in the anti-narcotics fight, after US politicians had urged him to do so.
He told reporters on Friday that he would repeat that plea to Beijing: "In a very respectful manner, we are going to send this information to reiterate the request that they help us."
Mexican Navy Secretary Rafael Ojeda said the container intercepted in Lázaro Cárdenas had packages weighing 34-35kg (75 pounds) with traces of fentanyl and methamphetamine hidden in fuel resin. The cargo had left the Chinese city of Qingdao and passed through Busan in South Korea before reaching Mexico.
Fentanyl is up to 50 times more powerful than heroin. The US Drug Enforcement Administration says 67% of the 107,375 US deaths from drug overdoses or poisonings in 2021 were linked to fentanyl or similar opioids. Fentanyl is linked to more deaths of Americans under 50 than any other cause, the DEA says.
The US authorities blame Mexican drug gangs for supplying fentanyl to users across the US. Last month three sons of drugs kingpin "El Chapo" - members of Mexico's Sinaloa cartel - were charged in the United States with fentanyl trafficking, but only one of them is in custody. Their father Joaquín "El Chapo" Guzmán is serving a life sentence in the US.
President López Obrador has said fentanyl is not produced in Mexico but bought by the drugs gangs from suppliers in Asia.
Last month Chinese foreign ministry spokesperson Mao Ning said "there is no such thing as illegal trafficking of fentanyl between China and Mexico".
"China has not been notified by Mexico on the seizure of scheduled fentanyl precursors from China," she said. Drugs listed in schedules are subject to various official restrictions.
Mao Ning said the widespread fentanyl abuse in the US was a problem "completely 'made in USA'". (BBC)
