The US National Hurricane Centre warned of "large and destructive waves" as Maria came ashore near Yabucoa on the southeast coast.
Puerto Rico's Governor Ricardo Rossello has told residents to brace for "the worst storm of the last century", opening 500 shelters that can accommodate 67,000 people.
"The wind sounds like a woman screaming at the top of her lungs!" photographer and storm chaser Mike Theiss posted on Twitter as the hurricane hit.
"We are getting absolutely hammered right now."
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"Puerto Rico being hit hard by new monster Hurricane," tweeted US President Donald Trump.
"Be careful, our hearts are with you -- will be there to help!"
Maria made landfall as a Category Four storm on the five-point Saffir-Simpson scale, packing winds of 155 miles per hour.
The US and British Virgin Islands -- still struggling to recover from the devastation of Hurricane Irma -- are also on alert, along with the Turks and Caicos Islands and parts of the Dominican Republic.
"I'm not denying I'm scared," said Noemi Aviles Rivera, a 47-year-old schoolteacher in Puerto Rico who experienced Hurricane Hugo in 1989 and Georges in 1998.
"I feel worried because it's the first time I'll see a hurricane of this magnitude."
Governor Rossello tweeted that more than 11,000 people had sought shelter already, with nearly 600 pets in tow.
In the US Virgin Islands, locals reported horizontal rain and trees swirling in the wind.
"We can hear debris banging on the aluminium windows as well."
In Guadeloupe, one person was killed by a falling tree as Maria hit, while another died on the seafront.
At least two more are missing after their boat sank off the French territory, while some 40 per cent of households in the archipelago of 400,000 were without power.
On neighbouring Dominica, Prime Minister Roosevelt Skerrit posted on Facebook yesterday that there were initial reports of "widespread devastation".
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