The grocer's premium Finest brand will be seen by millions of viewers when the fourth series of the ITV drama, set in a stately home, airs in the autumn.
Tesco announced its first TV sponsorship as it attempts to recover from the horsemeat scandal earlier this year and sliding sales and profits.
The multi-million pound sponsorship deal will last for a year and see the Finest brand appear across TV, mobile and online platforms.
Tesco UK marketing director David Wood said the award-winning show is "well-crafted, authentic and inspiring" - making it a "great brand fit" with the supermarket.
Mark Trinder, ITV commercial and online sales director, said: "The coming together of two of the UK's most high-profile brands this autumn in Tesco Finest and Downton Abbey makes for a high-impact partnership.
"The package is a great example of how partnering with ITV can enhance a brand's appeal to the benefit of our customers and viewers."
Tesco is under pressure to revive flagging sales which fell 1% on a like-for-like in the three months to the end of May.
Its Finest range was launched in 1998 and spans 1,400 premium products such as beef carvery rib and strawberry Eton Mess.
Shore Capital analyst Clive Black said Tesco is trying to "demonstrate it is more aspirational", ahead of an expected relaunch of Finest in coming weeks.
He said: "It's one piece of a jigsaw that Tesco is building at the moment to improve its lot.
"It's a highly-regarded TV series watched by a large body of people and shows a degree of self-confidence in the belief of Tesco.
"Management has dusted itself down and had a clear look at how it's perceived and how it needs to improve the perception."
Tesco replaces cruise ship operator P&O as the show's sponsor. The fourth series will run for eight episodes and follows the lives of a family of aristocrats and their servants.
The third series of Downton Abbey was watched by an average of 12 million viewers - claiming a 40% share of the audience - and aired on Sunday evenings.
The series is set at Highclere Castle in Berkshire and written by Julian Fellowes.
Angela Porter, head of brand advertising at Tesco, said: "Just like Downton, Finest has carved out its own unique place in British culture.
"During the economic downturn, we have seen people turn to Finest to treat themselves, especially as many aren't eating out so frequently."
Rival Morrisons sponsored the recent series of Britain's Got Talent, and Sainsbury's is sponsoring British athletics.