Sgt John Speed was gunned down near the former Leeds Parish Church on Kirkgate in the city centre by David Gricewith on October 31, 1984.
The 39-year-old was shot after going to the aid of colleague Pc John Thorpe, who had been fired on during a routine check on two men acting suspiciously near the church.
Pc Thorpe, 33, who was first at the scene, had spoken only a few words when one of the men pulled out a handgun and opened fire.
Gricewith, a garage owner and well-known criminal, was only identified as the gunman after his own death two years later.
A memorial was later installed at the scene of the incident.
In a post on Facebook, the Leeds outer east neighbourhood policing team wrote: "Today we remember Sgt John Speed. Murdered protecting the community of Leeds on this day in 1984 #lestweforget."