The Love Actually star will have a supporting role in the spy film, according to The Hollywood Reporter.
Armie Hammer, Alicia Vikander and Man of Steel actor Henry Cavill will also feature in the adaptation of the TV series, which ran from 1964 to 1968.
Grant, who most recently starred in Cloud Atlas with Tom Hanks, will play the head of British Naval Intelligence.
The film follows special agents Napoleon Solo, played by Cavill, and Illya Kuryakin - Lone Ranger star Hammer - who work for the mysterious United Network Command for Law Enforcement (U.N.C.L.E.).
Swedish actress Vikander, known for her role as Kitty in Joe Wright's Anna Karenina, has been cast as the female lead, while The Great Gatsby's Elisabeth Debicki will also star in the adaptation.
Tom Cruise had been tipped for the role of Solo, originally portrayed by Robert Vaughn in the TV series, while George Clooney was set to play the lead role in director Steven Soderbergh's earlier version of the film.
However, Clooney withdrew from the film and Soderbergh subsequently dropped out of the project.
Director Ritchie will co-write the script with Lionel Wigram, who wrote both of Ritchie's Sherlock Holmes films, starring Robert Downey Jr.
Production is due to begin in September with a release planned for 2014.
There were 105 episodes of the multi-award winning TV show, The Man From U.N.C.L.E, with the first series broadcast in black-and-white.

Commentators on the internet have been quick to make fun of Mr Boehner over the years, but his willingness to show emotion has also endeared him to many.
His latest crying bout came on Thursday as he watched Pope Francis address Congress and a crowd of thousands just outside of his office.
With his face scrunched, his wet green-blue eyes glistening and matching his tie, he did not disappoint.
Boehner, dubbed the "Weeper of the House," responded to jokes about his emotions with this tweet.
Other notable times he cried include when he became House Speaker, during Congressional Gold Medal ceremonies, at the unveiling of statues and even during an interview with the Golf Channel.
Here is how Twitter reacted to its favourite wailer unexpectedly announcing he will resign from Congress at the end of October:

Workie, as the animal is called, will star in a TV advertising campaign that starts on Wednesday evening.
Since 2012, 5.4 million workers have been signed up to a workplace pension, under the auto-enrolment programme.
But millions more people who work for small businesses, or individuals, have yet to be offered a pension plan.
All employers are being warned that they need to offer pensions to anyone who is over the age of 22 and who earns more than Â£10,000 a year.
That includes, for example, nannies, care workers, plumbers and bar staff.
The pensions minister, Baroness Altmann, was said to be "personally involved" in designing Workie.
A spokesperson for The Pensions Regulator said Workie was the embodiment of something you cannot ignore.
Hugh Grant has been cast in Guy Ritchie's big-screen adaptation of the 1960s TV show, The Man From U.N.C.L.E.
[NEXT_CONCEPT]
With John Boehner's resignation, Twitter is losing one of its favourite public figures, a politician famous for his tears and his perma-tanned complexion.
[NEXT_CONCEPT]
A ten foot tall hairy monster - reminiscent of Sweetums in The Muppets - is to be used to persuade more employers to set up a pension scheme.