Alhassan signed with Portiguese top-flight side but the deal did not go through at the last minute.
The league newcomers have announced his signing on a four year deal on their official website.
The FC Hearts starlet and former Flying Eagles and Golden Eaglets player was reportedly a subject of interest by the president of CD National.
Alhassan Ibrahim spent the whole 2017/18 season on loan at FK Austria Wien from Nigerian Professional Football League side (NPFL) side Akwa United.
The 21-year-old is regarded as one of the shining stars of the NPFL nicknamed Muazzam, and he has gathered valuable experience playing in the Europa League for Wien.
Clube Desportivo Nacional will hope Alhassan’s experience in Europe where he made over 20 matches for Wien in the league will help him adjust faster to the playing style in Portugal.
Alhassan graduated from FC. Heart, and played for Wikki Tourists before his move to Akwa United, he has made a total of four appearances for the Super Eagles.