Doordeals has extended its 'back-of-shirt' sponsorship with United for a third successive season.
Their brand and logo will appear on the club's 2018/19 playing kit for competitive games as well as supporter replica shirts.
Doordeals Operations Manager, Martin Fox, said: "We have been delighted to partner with United and we look forward to another season proudly supporting the Blades on and off the pitch.
"Our first season as shirt sponsors couldn't have gone better, with Chris Wilder's side comfortably winning the League One title, our second (the 2017/18 season) has been a strong, competitive effort in the Championship and we are looking forward to another successful season next time around."
Throughout the season Doordeals will run several social media competitions for Blades fans to win prizes, along with discounts on products.
Martin continued: "As a club sponsor we have thoroughly enjoyed working with Sheffield United and their commercial department, interacting with other supporters and of course following the Blades home and away.
"Our links with the club over multiple years have undoubtedly helped in raising our profile both locally and nationally."
Set up by managing director Steve Naylor in 2007 Doordeals is considered one of the UK's leading independent online timber door retailers.
United's Commercial Key Account Manager, Liam Robinson said: "Over the last two years, Doordeals has grown to become a household name with Blades up and down the country.
"We are excited to continue and build upon this relationship and aim to bolster the growth of Doordeals seen over the past 24 months. We are proud to continue to associate ourselves with a business that remains to both be nominated for and win prestigious accolades."
Today Doordeals operates from offices, showroom and distribution centre on Saxon Road, Heeley, supplying the retail and trade market from its large stock of popular sizes and styles of quality internal and external doors, as well as related accessories and ironmongery.
The business continues to grow and was crowned SME of the Year at the 2017 Sheffield Business Awards. The business triumphed at the ceremony. Judges recognised the firm for 'achieving consistent growth backed by a strong financial performance, while demonstrating a clear understanding of the markets that it operates in'.