A new plaque will also be revealed on the World War Two monument at the Guildhall Square paying tribute to Jean Louth and her 27-year campaign to see the memorial built.
The Blitz saw major British towns and cities, including London, targeted repeatedly by the Luftwaffe from September 1940 to May 1941.
It is estimated more than 43,500 civilians were killed in the raids and entire neighbourhoods destroyed.
In Portsmouth, significant landmarks, including the Guildhall and the Royal Garrison Church, were severely damaged.
Steve Pitt, leader of Portsmouth City Council said: “As we gather to mark the 85th anniversary of the January Blitz, we honour the strength and resilience shown by the people of Portsmouth during one of the darkest moments in our city’s history.
“These commemorations remind us of the importance of coming together to remember, reflect and ensure future generations understand the sacrifices made.”