John came ashore near Marquelia in Guerrero state, packing maximum sustained winds of around 120 miles (195 kilometers) per hour, the US-based National Hurricane Center said.
"Damaging hurricane-force winds, life-threatening storm surge and flash flooding are ongoing," it warned.
"Slow-moving Hurricane John will bring very heavy rainfall to coastal portions of southwest Mexico through the upcoming week," according to the NHC, which put John in the third-highest category on the five-step Saffir-Simpson scale.
"This heavy rainfall will likely cause significant and possibly catastrophic, life-threatening flash flooding" in the southern states of Chiapas, Oaxaca and Guerrero, it said.
A hurricane warning was in effect from east of Acapulco to Bahias de Huatulco on the Pacific coast.
"After landfall, the system will rapidly weaken over the high terrain of southern Mexico," the NHC said.
President Andres Manuel Lopez Obrador earlier warned people living along the affected coastline to be prepared.
"Seek higher ground, protect yourselves and do not forget that the most important thing is life; material things are replaceable," he wrote on social media platform X.
The National Civil Protection agency issued a red alert, telling people to stay indoors and keep away from windows.
Authorities in Oaxaca said they were opening temporary shelters, suspending school classes, closing beaches and mobilizing machinery in case needed to clear roads.
The international airport in the tourist resort of Puerto Escondido suspended all flights.
In Guerrero, authorities said around 300 temporary shelters were ready if needed.
Restaurant workers were seen bringing furniture in from beaches, while fishermen returned to shore.
Hurricanes hit Mexico every year on both its Pacific and Atlantic coasts, usually between May and November.
In October last year, Hurricane Otis, a scale-topping Category 5 storm, left a trail of destruction and several dozen people dead after slamming into the beachside city of Acapulco in Guerrero.
Otis rapidly intensified within hours from a tropical storm to the most powerful category of the five-step Saffir-Simpson scale, taking authorities by surprise.
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