The plaque was unveiled during a special ceremony, organised by Leeds Civic Trust and supported by the David Oluwale Memorial Association, on Leeds Bridge at 5pm yesterday.
The event concluded at 7pm and police believe the plaque was taken sometime between 7.30pm and 10pm.
David Oluwale had travelled to the UK from Nigeria in search of a better life, but was tragically targeted because of his mental health, homelessness and race, before his death in 1969.
The plaque followed the dedication of a new bridge, now named the David Oluwale bridge, which was recently lowered into place across the River Aire, connecting Sovereign Street to Water Lane.
Leeds Civic Trust said the people responsible for the theft had brought "shame to our city".
And West Yorkshire Police have now confirmed they are treating the incident as a hate crime, as they launch an investigation to find the culprits.
Leeds District Commander, Chief Superintendent Damien Miller said: “It is truly appallingly that someone would remove the plaque commemorating the life of David Oluwale, and we recognise the significant impact that this act will have had on all those involved in keeping David’s memory alive and on the wider community.
“The timing clearly suggests that this has been a deliberately targeted act and we are classing this as a hate crime.
“We are treating this incident very seriously and have detectives from Leeds District CID carrying out extensive enquiries to identify who is responsible and to locate and recover the plaque.
“Leeds Bridge is in a busy area in the heart of the city centre and we would urge anyone who witnessed any suspicious activity or who has any information that could assist the investigation to contact us immediately.”
Anyone with information is asked to contact Leeds District CID via 101 quoting crime reference 13220222118 or online via the Live Chat.
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.