Palmer, 65, was shot dead in the garden of his home in South Weald, Essex, in June last year.
Essex Police said the shooting had "all the hallmarks" of a professional hit.
Palmer's death was featured on BBC Crimewatch on Thursday, when his family offered the reward to catch the person or people responsible.
Once described as Britain's richest criminal, Palmer - whose nickname derives from his connection to the 1983 £25m Brink's-Mat gold bullion robbery - was jailed in 2001 for a £20m timeshare fraud.
At the time of his death, he was due to stand trial in Spain on charges of fraud, firearm possession and money laundering.
Speaking on Crimewatch, his partner Christina Ketley said: "There may be some people out there who don't have a tremendous amount of empathy towards our situation or the loss of John, but there is someone out there who is prepared to assassinate someone."
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Palmer's son James Ketley described how he rushed to help his father after his girlfriend found him collapsed in the garden on 24 June last year.
"I heard no gunshots, it was just very, very quiet. I could still feel his heart beating so I thought there might be a chance we could keep him alive.
"I felt so helpless because I had to watch him die in front of me."
Det Ch Insp Stephen Jennings, from Essex Police, said Palmer's family had independently offered the reward in the hope of catching his killer.
He said the force could not comment on rumours Palmer had turned informer about his previous criminal connections and links to other organised crimes, including the Hatton Garden raid.
"Due to John's significant criminal history, there are people or groups of people who may have wished to do him harm.
"Therefore our search is not just for the gunman but for a person or group of people who may have commissioned the killing."
Police confirmed two officers were disciplined for failing to spot Palmer had been shot when they arrived at the scene, instead believing he had died from natural causes after recent keyhole surgery to his chest.
A £50,000 reward has been offered to help catch the killer of notorious criminal John "Goldfinger" Palmer.