Former Turkey Under-19 international Ciftci, who can play through the centreor out wide, has penned a two-year contract after impressing manager Jackie McNamara on a three-week trial spell.
The 21-year-old, who made 19 appearances for Portsmouth, was available on a free transfer after leaving NAC Breda in his native Holland.
In a club statement, McNamara said: "Nadir has really impressed me. He will become an important member of our squad.
"He is strong, clever, has pace and an eye for goal. He has quickly become part of the squad and is a great guy to have around the dressing room.
"His desire to sign for Dundee United was obvious because he had more lucrative options but made it clear he wanted to progress his career at Tannadice."
Ciftci, United's ninth summer signing, added: "This is a fantastic club with great players and coaches. I want to join Dundee United because I know I can become a better player here.
"The players are not only good footballers but also have made me feel so much part of the club since I arrived. The fans have also made me feel welcome. I am delighted to have signed."
United earlier announced that both McLean and Ryan had left by "mutual consent" and "with the best wishes of all at the club".
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- Dundee United