The striker registered four goals for the Hornets in his 25 appearances as they managed to stay in the English top-flight having finished on the 17th spot last season. He has featured four times for the English side this term before making his switch.
The Pepineros announced the arrival of the former Malaga talisman on Friday.
“Leganés Sports Club and Watford Football Club have reached an agreement for the one-year assignment of thirty-year-old Moroccan striker Nordin Amrabat,” the club said.
Amrabat will be expected to make his debut for the Estadio Municipal de Butarque outfit when they clash with Getafe on September 8.