Security minister Luis Cordero said the suspect used a liquid accelerant to start fires in a wheat field, with authorities seizing five litres of fuel from him.
He was arrested at dawn in the town of Perquenco in the Araucanía region, south of Biobio.
The fires began simultaneously on Saturday in various parts of the Biobio and Nuble regions, about 500km (310 miles) south of the capital Santiago.
Fanned by strong winds and high temperatures, the flames quickly ripped through the coastal towns of Penco, Lirquen and Punta de Parra, leaving a blackened landscape of smouldering ruins.
Interior minister Alvaro Elizalde told a press conference on Wednesday that an estimated 20,000 people suffered property damage from the fires, including some 800 homes that were destroyed.
President Gabriel Boric visited Biobio on Wednesday, where he said, “We’re working with heavy machinery to clear streets and remove debris, and we continue fighting the fire.
“We’re still in a state of emergency,” he added.
Other fires were later reported further inland, in the Biobio town of Florida, about 40km (25 miles) east of the city of Concepcion and in Araucania.
Cordero said substances used to start fires, including plastic containers containing accelerant, were found in Concepcion.
Firefighters were still battling 35 blazes Wednesday – 22 in Biobio, five in Nuble and eight in Araucania, according to national forestry officials.
A drop in temperature in recent days has helped ease the situation.
“We managed to reduce the intensity of the fire,” Carlos Zulieta, a firefighter in Florida told AFP, adding that it was now advancing “more slowly”.
The government said it would pay compensation of US$700 to US$1,500 to victims.
Aid began trickling into affected areas on Wednesday.
Municipal workers and private companies were delivering portable toilets and generators to Lirquen, where some families are camped out in the ruins of their homes.
In February 2024, wildfires broke out around the coastal resort of Vina del Mar, 110 km from Santiago, leaving 138 dead.
Investigations revealed that firefighters and forestry brigade members started the fire, which spread rapidly due in part to high temperatures during the southern hemisphere’s summer.