The former Sheffield United midfielder began his career at home-town club Luton before signing for the Blades, where he made four first-team appearances.
Following short loan spells at Burton and Alfreton, Harriott arrives at the Sixfields Stadium much to the delight of his new boss.
"Matty will increase the competition in central midfield," said Aidy Boothroyd.
"He is strong, he can be physical and he has a good engine. He can play as well and he is really hungry and desperate to be given a chance.
"He did well for us in pre-season and I think he will be a good addition."