Otis was blowing maximum sustained winds of 257km/h and was expected to bring heavy rain capable of "catastrophic damage," the US National Hurricane Centre said.
Otis was about 88km south-southeast of Acapulco, the Miami-based NHC added.
Once Otis comes ashore, it should weaken rapidly, it said.
Hurricane conditions were expected within 12 hours for the stretch of coast between the beach towns of Zihuatanejo and Punta Maldonado in the state of Guerrero, which is home to Acapulco, the NHC said.
The storm could bring up to 51cm of rain in parts of Guerrero and neighbouring Oaxaca state, possibly causing flash flooding and mudslides, a "potentially catastrophic" storm surge, and surf and rip currents, the NHC added.
Schools across Guerrero cancelled classes for Wednesday ahead of Otis' arrival, Governor Evelyn Salgado said on social media.
Over the weekend, Hurricane Norma killed at least three people as it passed along the northwest coast of Mexico. Days before, the powerful Hurricane Lidia left one person dead and several injured after battering Mexico's Pacific coast.
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