The Kings Park blaze, previously rated as "watch and act", was downgraded after it became contained and controlled, authorities said.
"The majority of Kings Park has reopened to the public, subject to weather conditions over the next 24 hours, and people are advised to stay alert to their surroundings if they enter the area," EmergencyWA wrote online.
There is currently no threat to lives or homes.
Fire crews were called to the park, which abuts Perth Children's Hospital, yesterday afternoon after five separate blazes were called in within 30 minutes.
WA Today has reported police are investigating the suspicious fires.
