The entire island was hit by a power outage on Sunday.
Over 1.4 million people have been affected by the power outage, which is all electricity customers, writes the Washington Post.
Work is now underway to restore power and according to the current forecast it may take several days.
Hurricane Fiona struck two days before the anniversary of Hurricane Maria, which crippled the island in 2017 and caused almost 3,000 deaths.
US President Joe Biden has classified the situation in Puerto Rico as an emergency.
This means that the US disaster management agency gets the opportunity to coordinate disaster relief and contribute with protective measures.
Other islands in the Caribbean Sea are also threatened by the hurricane.
The greatest danger concerns the Dominican Republic and Haiti.
The US hurricane authority NHC warns that the hurricane may increase in strength during Monday.