The 31-year-old Senegal international trained with Neil Warnock's side this week and his signing was confirmed ahead of Saturday's Capital One Cup clash with Shrewsbury at Elland Road.
Warnock seems to have forgot the bad blood that existed between himself and Diouf during an FA Cup match between Blackburn and QPR in January, 2011. The then QPR manager described Diouf as a "sewer rat" following a collision between his winger Jamie Mackie and Gael Givet that wound up with the Scotland player being carried off with a broken leg .
"He’s broken his leg and Diouf was calling him all sorts. Even the Blackburn people were embarrassed," said Warnock at the time.
“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.”
The controversial player has made over 300 appearances in English football, after starting his career in France. He spent nine years in the Barclays Premier League with Liverpool, Bolton, Sunderland, and Blackburn.
He also had a loan spell with Rangers before joining Doncaster in the Championship last season.
Diouf, who has twice been named African Footballer of the Year, made 23 appearances for Doncaster, but could not prevent the Yorkshire side from being relegated from the Championship.