The Glee actress and Stan, who was in a relationship with Leighton Meester for two years, are reportedly smitten with each other.
"He came to Dianna's 3pm Glee performance at the Staples Centre in Los Angeles on May 28," a source told Life and Style.
"He waited for her backstage. Once the cast finished the show, Dianna ran backstage to Sebastian, and they were all over each other! Dianna and Sebastian love hanging out together. He can definitely hang with the Glee cast."
Agron was linked to Stan's Captain America: The First Avenger co-star Chris Evans in April after meeting him at a pre-Oscars party in February.
An insider said at the time: "Chris's brother Scott is a huge fan of her show, and he particularly loves Dianna. He and his friends like to joke that as long as she stays on the show, the family will welcome her with open arms!"