The 19-year-old - who made one brief appearance for Lens in their last match in Ligue 1 last season - had his original ?250,000 transfer blocked by FIFA last September but he was still allowed to go on loan.
However, Lens issued a statement saying that Situ was now fully the property of Swansea.
"Lens and Swansea (Premier League) reached agreement on Tuesday concerning the permament transfer of Darnel Situ, who since the beginning of the season has been on loan at the Welsh club," read the statement.
Swansea have enjoyed a respectable first half of the season on their maiden appearance in the Premier League - though they did compete in its previous entity the First Division - presently lying 12th seven points above the relegation places.
Source: AFP
Source: AFP