Christiansen joins the club from APOEL Nicosia in Cyprus after winning the First Division title last season, as well as leading the club to the last 16 of the UEFA Europa League.
Prior to his spell at APOEL, the 44 year-old secured AEK Larnaca two second place finishes in the Cypriot First Division, the highest position in the clubs history.
Born in Denmark, as a player Christiansen started his career with Barcelona and also had spells at Villarreal and German outfit VFL Bochum.
With the option to play for Denmark or Spain at international level, he opted for the latter, winning two caps for Spain and scoring one goal.
Speaking about the appointment, Leeds United Managing Director, Angus Kinnear said: “We are delighted to make this announcement today as it successfully concludes what has been a thorough and extensive search, following a huge level of interest from all over the football world.
“We wanted to appoint someone who can help us create a winning culture at the club and unite everyone connected with Leeds United, from the players to the supporters, ultimately taking the club back to where we all feel we belong.
“Thomas was someone who had been on our radar from the very early stages of the process and quickly established himself as the outstanding candidate.
"We are confident we have found a man with who can work with us to take the club to the next level.”
Christiansen will face the media on Monday and his press conference will be broadcast live on the club's official Facebook page, his first exclusive interview with LUTV will be available online on Monday afternoon.