The ex-wife of Tom Cruise was spotted at Queens Supreme Court watching the trial of a 25-year-old man accused of shaking his baby to death in 2007.
According to Gossip Cop, Holmes "listened intently" to the opening argument of prosecutor Leigh Bishop, believed to be a friend.
The Batman Begins actress is said to have "left the courtroom without fanfare" soon afterwards.
Katie Holmes with Tom Cruise
Holmes has remained in New York following the closure of her Broadway play Dead Accounts on January 6. The critically-panned production ran for less than six weeks.
Katie Holmes leaving a performance of Dead Accounts
The 34-year-old was recently forced to deny rumours that she is dating Jake Gyllenhaal, after it was claimed that the pair had been set up by her former Dawson's Creek co-star Joshua Jackson.
Cruise and Holmes reached a divorce settlement in August 2012 after five years of marriage.