Each one has been painted black, decorated with a gold trim and features an image of a notable black Briton or a piece of work by them.
The London postbox can be found in Acre Lane, Brixton, near to the Black Cultural Archives.
It features the image “Queuing at the RA” by Yinka Shonibare, who is one of six artists that were commissioned by Royal Mail to produce original artworks for a set of special stamps issued to mark the 250th anniversary of the Royal Academy (RA).
“As a citizen of the Commonwealth, it was particularly important to me to be making a visible contribution in a historic public space,” Mr Shonibare said.
Footballer Walter Tull, who became the first black player to sign for Rangers, before he was killed in action in the British Army in 1918, appears on the Glasgow postbox in Byres Road. He had played as a forward with Spurs and Northampton Town.
Tull previously featured in a set of stamps released in 2018 to mark the centenary of the end of World War One and was also the first black Army officer to command troops in a regular unit.
Mary Seacole, a Jamaican-born nurse who cared for wounded British soldiers during the Crimean War, features on the Cardiff postbox, which can be found in King Edward VII Avenue, while stand-up comedian and co-founder of the Comic Relief charity, Sir Lenny Henry, is honoured by a postbox in Bedford Street, Belfast.
Anyone wanting to find out more about the figures featured on the postboxes can scan a QR code on the outside, which will direct them to an online gallery on the Royal Mail website.
The postboxes will remain in place until the end of October.
Peter De Norville, Royal Mail's head of diversity and inclusion, said: “Black History Month is a great opportunity to celebrate the contributions that black people have made to this country over many generations.
“We are also using it as an opportunity to celebrate the vital work that our black employees do throughout the nation, from the mail bag to the meeting room.”