It will be known as Stephen Lawrence Day.
The announcement comes on the 25th anniversary of the death of the 18-year-old, who was stabbed to death by a racist gang.
A memorial of Lawrence’s life and legacy took place today.
Lawrence’s mother, Doreen Lawrence, and his brother Stuart greeted guests at the church entrance before the service.
Stephen Lawrence’s father Neville said the annual national commemoration of the murdered teenager’s life is “a mark of what we have been trying to do for years – our son’s memory is going to be enshrined in history”.
Prince Harry and his fiancee Meghan Markle joined the family and other notable figures at St Martin-in-the-Fields in Central London for the event.
Mrs May, Labour leader Jeremy Corbyn and London Mayor Sadiq Khan were among the 800 people in attendance.
Prince Harry read a message of support on behalf of the Prince of Wales, who in 2000 gave the annual Stephen Lawrence Memorial Lecture which began with a tribute to the Lawrence family.
During the event Sir Lenny Henry interviewed three young beneficiaries of the Stephen Lawrence Charitable Trust.
The young murder victim was attacked in Eltham, south-east London, on April 22 1993.
David Norris and Gary Dobson, two of the group of up to six assailants, are serving life sentences for the racially-motivated killing.
Three other men have consistently been accused of the killing but never convicted.
A sixth gang member was thought to be involved in the attack.
Last week, Neville Lawrence, announced that he would consider talking to his son’s killers and had decided to forgive the gang of racists.
He and his former wife, who is now Baroness Lawrence, have campaigned for more than 20 years to get justice for their son.
The murder of the teenager has become a watershed moment in modern race relations in the UK.
Duwayne Brooks, who was with Lawrence during the attack, says he heard the group yell racially abusive language before launching their violent assault on his friend.
A report into the case concluded the police made mistakes and were guilty of “institutional racism”.
The bungled case led to a major public inquiry and eventually a change in the law to allow Dobson to be tried twice for murder.
The Met has admitted the investigation is unlikely to progress without new information.
But focus on the case will continue with an inquiry into undercover policing, probing claims that police moles infiltrated campaign groups supporting the Lawrence family.