"Modern Family" star SARAH HYLAND left a cast party in Sydney, Australia, after a fan she was posing with grabbed her breast.

The show is filming a holiday episode in Australia ... and Sarah was posing with fans outside the event on Tuesday night, when a 29-year-old guy asked for a photo ... then GROPED her.

Sarah pushed the guy away and yelled, "Don't touch me there!"  Then she left in tears.  The groper was arrested.  

She later Tweeted an apology to fans she didn't get to meet, saying she, quote, "had to leave due to an inappropriate touch of a fan."

READ THE STORY AND SEE PHOTOS OF THIS CREEP HERE