The Jamaica international, who arrived at the Riverside on loan last season, was sacked by Wigan after he was found guilty of sexual assault and actual bodily harm over an incident in a London nightclub, which occurred in December while he was on loan at Hull.
King spent a subsequent spell on loan at Middlesbrough before returning to Wigan in the summer, but he will now be left without a club after a mandatory notice period has expired.
King's agent, Tony Finnegan: "No one saw this coming in light of the evidence. He's very, very disappointed - because clearly, as he said in his evidence, it's mistaken identity. He didn't do that.
"He'll be devastated, sitting in a cell with someone tonight. But he's a grown man; he's been there before and he'll take it on the chin.
"No one expected this. I asked Marlon and he said 'I'm not guilty, Tony - I didn't do it. I want the British justice system to find me not guilty'."