The casino giant has previously sponsored Aston Villa and Swansea City, and has advertising hoardings at other grounds. For Leeds shirts, its familiar red insignia will be printed in blue.
Leeds executive director Paul Bell said: “We are delighted to announce a three-year shirt sponsorship deal with 32Red PLC, the UK’s premier online casino entertainment brand.
“We are hugely excited by this partnership and the opportunities it will deliver for all involved.
“32Red are a modern, forward-thinking brand with a long-standing and successful history of sponsoring high-profile teams within football and we are delighted to welcome them into our commercial partners programme.
“We look forward to building a strong relationship between our two brands both on and off the field as we seek to return to the Premier League.”
There was mixed reaction on social media among Leeds fans, for many of whom red invokes inter-Pennine rivalries. When the Elland Road branch of McDonald’s opened in 2002, red was absent from its fascia.
Michael Holdsworth tweeted this morning: “No problem with what’s on the front of our shirts at all, get a grip.”
But also on Twitter, Oliver Williamson posted: “You’ve put the word “red” on the front of our shirts.”
The Scratching Shed website tweeted: “Pleased they had the good sense to colour the logo blue. Shows proper customer research.”
Twitter user @ephemeraljoy, meanwhile, tweeted the new sponsor directly and said: “Can you change your name to 32 White, Yellow and Blue?”
United spent this season just ended without a name on the front of their shirts following a dispute with previous sponsor Enterprise Insurance.