The Senegalese winger, who has courted criticism from the west Yorkshire club's manager Neil Warnock, has moved to Elland Road on non-contract terms
The 31-year-old Senegal international, who was twice named African Footballer of the Year, had been training with the west Yorkshire side this week.
Diouf, whose former clubs also include Blackburn and Sunderland, has made over 300 appearances in English football and spent much of last season with Doncaster Rovers, playing 23 times as they were relegated to League One.
His move to the Elland Road club will come as a shock to some as current manager Neil Warnock once labelled Diouf ‘a sewer rat’, following his tackle on QPR striker Jamie Mackie that left the Scot with a broken leg.
The winger also berated Mackie as he was stretchered off, further incensing the former Hoops boss.
"For many years I've thought he is a gutter-type of boy and I was going to call him a sewer rat but that might be insulting the sewer rats. I think he's the lowest of the low," Warnock said at the time.