The Serie A giants have allegedly taken up the option to buy the 25-year-old outright, despite the fact that he has spent most of his spell at San Siro on the sidelines
The Colombia international signed for the Nerazzurri in January on a six-month loan deal after becoming a fringe figure at the Estadio do Dragao under Vitor Pereira during the first half of the season.
A calf problem saw Guarin spend the first three months at San Siro on the treatment table, but he was able to impress in his six subsequent appearances for Andrea Stramaccioni's side.
Italian sources claim that Inter have spent €12.5 million on Guarin, although his agent remained coy on the exact fee, but stated that the Nerazzurri had been tracking his charge for a long time.
"I cannot talk about figures," Marcelo Ferreyra told Maisfutebol. "I can only say that Guarin is very satisfied, happy with this opportunity at Inter and extremely grateful to Porto.
"These talks with Inter had been taking place for two years. This is the best for Porto, Inter and Fredy.
"He is very grateful to Porto and especially to president Pinto da Costa, who gave the green light to the transfer, knowing that he has received a big sum."
The 25-year-old midfielder, who joined Porto from Saint-Etienne in 2008 for an undisclosed fee, rose to prominence in 2010-11, scoring eight goals during the second half of the season under Andre Villas-Boas.