The 26-year-old Brazilian is being viewed as a direct replacement for Kwadwo Asamoah, who is said to be on the verge of joining Juventus
The move was announced on the Zebrette's official website on Thursday evening, but the club did not reveal any details relating to the transfer or the length of the player's contract.
The 26-year-old's arrival at Stadio Friuli is significant, though, as Udinese are said to have agreed to sell midfielder Kwadwo Asamoah to Serie A champions Juventus.
The Brazilian, who earned the moniker 'Pitbull' because of his tenacity in the middle of the park, began his career with Santo Andre but has spent the past three years at Flamengo.
Willians' signing comes just under a fortnight after Udinese announced the acquisition of Caen centre-half Thomas Heurtaux.
Udinese finished third in Serie A in 2011-12, and will compete in the Champions League qualification play-off round for a second consecutive year.