Jurgen Klopp has been tracking Martins' progress with a view to bolstering his already formidable attacking options further. Martins name was mentioned in relation to Liverpool last summer before they signed Mo Salah from Roma instead.
However, AS is reporting that Martins has agreed to join Atletico, becoming the latest Sporting player to depart the club after the club's training ground was invaded by angry supporters in the final weeks of the 2017/18 season.
Several Sporting players terminated their own contracts after that incident and Martins is among them, meaning that Atletico will be able to get him on the cheap.
The transfer will complete the set of major summer signings for Diego Simeone, with Thomas Lemar, Nehuen Perez and Rodri all having moved to the Wanda Metropolitano already.
Martins will be the right winger that Atletico have been lacking since Yannick Carrasco moved to China in February. Koke and Saul Niguez have both been used in that role since.
The Portuguese winger played in both legs of Sporting's Europa League defeat to Atletico in April, but Simeone was obviously impressed with his performances.
Martins was part of Portugal's 23-man squad at the World Cup, but he only made one appearance, coming off the bench in the group game against Morocco.