Ms Swift, 27, accuses broadcaster David Mueller of slipping his hand under her dress while they posed backstage together at one of her 2013 concerts.
Her lawyer said in his opening statement at the court in Denver, Colorado, that Mr Mueller had "grabbed her rear end".
Mr Mueller sued the singer in September 2015, calling her allegation false.
He said she had cost him his $150,000-a-year job as a host at country station KYGO-FM.
On Tuesday, jurors were shown a photograph of the alleged assault at a VIP room meet-and-great before the gig on 2 June 2013.