The Sweden international defender has been a big hit at Elland Road since arriving on a season-long loan from Torino.
Now the 6ft 5in 25-year-old, who sealed Wednesday’s 2-1 win at Blackburn Rovers, has committed his long-term future to the club.
Jansson has agreed a permanent switch for the summer, when he will begin a three-year deal.
“I had a good feeling when I came here,” he said. “Garry Monk remembered me from when he was at Swansea and I was at Malmo and we played against them.
“I remember the feeling when I saw Garry for the first time when I came to Elland Road.
“He came over and gave me a big smile and big hug. That was so important for me.
“In Italy the situation with the coach is a little different.”