While in Barcelona with her band The Pretty Reckless, the former Gossip Girl star put on a risqué performance for concertgoers, according to the Daily Mail.
Momsen is said to have straddled one of her female fans and allowed another to grope her breasts.
The 17-year-old is rumoured to be dating Paul Weller's son Natt after the two were spotted attending a music festival together.
The Pretty Reckless are currently in the midst of a European tour that will see them stop in Barcelona, Madrid and Lisbon this week.
In January, Momsen revealed that when she was a child she was taught that sexual intercourse is wrong, explaining that she now views the act as natural.
Momsen previously described herself as a workaholic, stating that she is fortunate to have her Pretty Reckless bandmates Ben Phillips, Mark Damon and Jamie Perkins with her to keep her company.
Watch a series of images of Taylor Momsen's performance in Barcelona below: