The 26-year-old makes the move following two-and-a-half seasons at Stadio Friuli, during which he scored three goals in 69 Serie A appearances.
The transfer of the former America Cali and Palmeiras star, who has 38 caps for Colombia, was announced on Napoli's website.
"The left winger will arrive on loan with the option to buy out half of his contract at the end of the season," the club said.
"Armero will undergo his medical tomorrow, after which the deal will be made formal."
The news comes as a big boost to coach Walter Mazzarri's offensive options, as he bids to claw back a seven-point gap between his third-placed side and Serie A leaders Juventus.