Police evacuated 115 residents on Wednesday from the facility, whose air conditioning was cut by the storm.
Broward County Mayor Barbara Sharief said three were found dead at the nursing home in the city of Hollywood. Two others died upon reaching hospital.
Ten million people are still without power in Florida, Georgia and the Carolinas after Hurricane Irma.
The storm - which has claimed more than two dozen lives in the US - struck southwestern Florida on Sunday morning as a category four hurricane before weakening to a tropical depression on Monday.
Irma earlier left a trail of destruction in the Caribbean, where nearly 40 people were killed.
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Hollywood Police Chief Tomas Sanchez said the Rehabilitation Center at Hollywood Hills facility has been sealed off and police are conducting a criminal investigation.
"It may be related to a loss of power in the storm," he said, adding that officers were checking in on 42 other assisted living facilities and nursing homes throughout the city.
Rescue crews saw "a number of people in respiratory distress" when they arrived at the Hollywood Hills facility on Wednesday morning, another official said.
The facility is not the only Florida nursing home that has been left without power by Irma.
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