The former X Factor judge had denied hitting her fellow festivalgoer in the VIP area during an incident at the Chelmsford music festival in August 2013.
But today at Stratford Magistrates' Court, she was found guilty by a judge.
District Judge Angus Hamilton said: "I am quite sure to the requisite standard of proof, based on my analysis of the evidence that I have heard, that Ms Contostavlos did strike Mr Morgan in the face during an argument and that that blow caused minor bruising and swelling."
She will have to pay £3,020 in total, including legal costs, a fine, compensation and victim surcharge.
Tulisa, 26, has vowed to appeal, insisting: "I did not assault Vas J. Morgan!"
Reading a statement outside court, she told reporters: "This has been a rollercoaster week for me. I never wanted any of this. I did not assault Vas J. Morgan and I'm outraged at this decision. I will be appealing it. There are a number of high-profile witnesses who were there and know what happened. I urge them to come forward and tell the truth at the appeal.
"I am determined to clear my name as I have already done once this week. I repeat - I did not assault Vas J Morgan. I am innocent."
Tulisa, 26, was pictured arriving at the court on Friday morning (25 July) dressed in a smart suit and high-necked white lace shirt.
She was accompanied by her PA Gareth Varey, who was cleared of using threatening behaviour at the festival.
Tulisa had been accused of being embroiled in a drunken altercation with celebrity blogger Vas J. Morgan while attending V Festival 2013 at Chelmsford in Essex.
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