Witnesses told police the arrested suspect helped reload a semi-automatic pistol as the gunman went house to house in search of people to attack in Kawit town outside the capital, Manila.
Police say John Paul Lopez was arrested in Imus town near Kawit late on Friday, hours after gunman Rolando Bae was killed by police.
Juanito Victor Remulla, the governor of Cavite province where Kawit is situated, says relatives led police to Lopez.
Lopez claims he was forced at gunpoint to help reload the pistol between the shootings.
Police spokesman Generoso Cerbo says they're yet to determine the full extent of Lopez's alleged involvement in the shootings, but that he will face criminal charges
He says 12 people are being treated for gunshot wounds in hospitals in Manila and Cavite following the rampage.
Police are examining drug use and a failed political campaign as possible motives for the shooting.
They say they found drug paraphernalia, an expired gun license and ammunition in the house of the slain gunman.
Police say Bae had been an elected member of the village council, but left the community after being defeated in a 2010 ballot for the post of village chief.
Bae and Lopez, described by police as the gunman's employee, reportedly began drinking and taking the banned stimulant methamphetamine on New Year's Eve.
Friday's shooting rampage followed the New Year's Eve deaths of two children by celebratory gunfire in Manila, which has triggered outrage and condemnation of the Philippines' poorly enforced gun laws.
There were 1.2 million registered firearms in the Philippines last year, with another 600,000 unlicensed weapons in circulation, according to police.