Posting a link to a fund-raising site on his Facebook page, Antonino ‘Tony' Ciaverello put out a call for financial help, saying he is in a “disastrous situation”.
His car parts business was seized by police last summer amid suspicion it was used to launder some of Riina's money.
“Unfortunately, I need to ask for help from those who can give me a hand, and I promise to pay back everything once I am in work again,” he wrote.
“They took everything, and now I don't have a job. To those who can help me, please do as I have three children. I am looking for a job but nobody gives me one. Thank you and may God bless you.”
The post, which has so far garnered eight ‘likes' on Facebook, was met with incredulity by some of his followers.
“You're a martyr? I'm sorry, but where were you when your father-in-law was killing people, throwing a baby into acid and hoarding money?” wrote one.
Ciavarello is married to Maria Concettato, Riina's eldest daughter. They live in the Sicilian town of Corleone, the historic stronghold of the Cosa Nostra mafia clan which was headed up by Riina.
Riina, nicknamed 'The Beast' because of his cruelty, has been serving a life sentence since 1993. He is thought to have ordered more than 150 murders and is also believed to have been behind the assassinations of the anti-mafia judges Giovanni Falcone and Paolo Borsellino in 1992.
In July, a request by the 86-year-old to be released from prison because he's seriously ill was rejected by a court on Bologna.
The court argued that Riina, who is suffering from kidney cancer, was still dangerous, noting that he had never disassociated himself from the Cosa Nostra and that there was a risk he could re-offend.
The ruling also ordered the seizure of a villa, plots of land and three businesses it said were fraudulently owned by Riina and his family.