Miller went to see Hartnett on Tuesday (March 31) at Cedars-Sinai Hospital in Los Angeles.
Hartnett was admitted to the hospital on Monday and is currently under observation for gastrointestinal problems.
"Sienna had just 24 hours to fly all the way to L.A. to see Josh. She was very concerned and wanted to be sure he knew how much she cared for him," a source told X17.
Miller and Hartnett met last year when the actor was starring in Rain Man in London.