The midfielder's temporary switch from Loftus Road to Vicarage Road will also have the option of becoming a permanent one if it is agreed.
Mali international Diakite joined Rangers from Nancy initially on loan in January 2012.
The move became permanent on a four-year contract at the end of the 2011/12 season but Diakite has found himself out in the cold at QPR this season.
His only appearance so far was in the 2-0 home defeat to Swindon in the Capital One Cup on August 27 when he was substituted in the 56th minute.
Watford reportedly turned their attention to Diakite after a move to bring in Ghana midfielder Derek Boateng on loan from Fulham stalled.