"I am sure that both parties will benefit greatly from this sponsorship deal, and next season promises to be an exciting one in the Coca-Cola Championship," said Iron chairman Steve Wharton.
Rainham Steel is based in Essex, but operate a £7million depot in North Lincolnshire.
"The company feels that the loyalty and commitment of its staff in Scunthorpe has contributed significantly to its success," said Rainham Steel chairman Bill Ives.
"We are very happy to continue investing in the Scunthorpe area, we are proud and pleased to be involved with Scunthorpe United."
Kit image reproduced by kind permission of Scunthorpe United FC.