The N-Dubz star hit Savvas Morgan during an altercation at V Festival in August last year.
She was found guilty by a judge at Stratford Magistrates' Court on Friday.
Her assistant, Gareth Varey, was cleared of using threatening behaviour during the incident.
After being convicted of the assault, Contostavlos was ordered to pay a total of £3,020.
This consisted of a £2,700 contribution towards the cost of the prosecution, a £200 fine, £100 compensation to Mr Morgan and a £20 victim surcharge.
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."
Contostavlos shook her head and smiled in disbelief as Friday's guilty verdict was read out.
Earlier this week, a drugs trial against Contostavlos collapsed at Southwark Crown Court after the judge said there were "strong grounds" to believe that undercover reporter Mazher Mahmood - known as the fake sheikh - had lied on the witness stand.
In a statement after Friday's guilty verdict, Contostavlos said: "This has been a rollercoaster week for me. I never wanted any of this.
"I did not assault Vas J Morgan and I am 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."
Mr Morgan - who has joined the cast of ITV2 series The Only Way Is Essex - said after the conclusion of the case: "I'm pleased we can finally put an end to this and move on.
"It's been a difficult time for everyone involved and I'd just like to put it behind me. I've had my character questioned and now feel vindicated by the verdict."