Richard Moss, 39, died from a brain tumour in February. His ashes were put in silver bracelets made for his four sons.
Chavez Robinson, 21, said the bracelet was of "massive sentimental value" and had been stolen from a van in Hull.
Police said it was investigating the theft of a van.
Mr Robinson said he was "absolutely heartbroken and gutted" to find his bracelet had been stolen.
"All my brothers had identical bracelets, it's all silver with gemstones. We have it on all the time to remind us of him and so we can be closer to him," he added.
"The first time I take it off to help a friend with some work - I put it on the seat in the van - and it's gone.
"I'm so angry and upset."
Mr Robinson said he was offering the reward for its safe return after the van was reported stolen on 16 June.
"It's not like it's a normal bracelet, you can't just replace it.
"It's the ashes inside that's of value."
Mr Moss, who lived in South Ferriby, North Lincolnshire, died after he was diagnosed with three brain tumours earlier this year.
In the days before his death, he wrote a personal letter to his family telling his sons to "keep strong" and "make the most" of their lives.
Humberside Police said the van, which was parked on Victoria Gardens, near Cottingham Road, had "since been recovered" and its investigation was ongoing.
A man is offering a £500 reward for the return of a bracelet containing his dad's ashes which was stolen three months after he died from cancer.