Just after midnight on Wednesday (06:25 GMT), it made landfall close to the well-known Acapulco resort, according to the National Hurricane Center (NHC).
As torrential rain batters the region, officials have issued warnings about a potentially fatal storm surge and the potential for landslides.
As it approaches land, the storm is already starting to lose strength.
Hours before Otis made landfall, David Hall arrived at the Princess Mundo Imperial resort in Acapulco for a business meeting. He claimed to the BBC that the rain and wind had harmed the structure.
According to him, the wind caused the hotel room windows to break and leak, removing everything from the room.
The building "shivered," according to Mr. Hall, who is from Colima, Mexico, which is around 600 kilometers (372 miles) away from Acapulco. He said this was similar to an earthquake.
He has been sheltering with hundreds of other hotel guests while the hurricane moves through its worst.
Many individuals are terrified, according to Mr. Hall.
In the state of Guerrero, a 350 km stretch of coastline between the coastal cities of Zihuatanejo and Punta Maldonado is under a hurricane alert.
Guerrero has already reported power shortages, according to Mexico's civil protection organization. at anticipation of the storm, all classes at schools across the state were canceled.
Unconfirmed footage that have been uploaded online depict water outside the Copacabana hotel and damage to an Acapulco hospital.
Waves as high as 10 meters (32 feet) were predicted to break on the coastlines of Guerrero and western Oaxaca state, according to the national water agency of Mexico. Mudslides have also been predicted.
People were advised to evacuate to emergency shelters and stay clear from ravines, streams, and rivers by President Andres Manuel Lopez Obrador.
Additionally, according to the NHC, Otis was predicted to deliver up to 20 inches (51 cm) of rain on Friday across Guerrero and parts of the western coast of the neighboring state of Oaxaca.
The rate at which Otis strengthened on Tuesday into a category five hurricane—the strongest storm classification—was unusual, according to scientists.
Meteorologist Philip Klotzbach said that it surpassed the record for the quickest intensification rate over a 12-hour period in the Eastern Pacific, attaining 80mph in that time.
Significant flooding was already seen earlier this month in several areas of Mexico's Pacific coastline with the passage of Tropical Storm Max. Two fatalities were reported by the local media in Guerrero as a result of the storm.
A few days later, one man was reported dead following the landfall of Hurricane Lidia in the state of Nayarit, which is located northwest of Guerrero.