The "Lethal Weapon" star offered hugs and moral support to the 375 workers who stand to lose their jobs next month if the plant shuts down.
After the tour, Glover held a brief news conference and appealed to Germany's Hugo Boss AG to reverse its shutdown decision.
Glover led a boycott of Hugo Boss formal wear at the Oscars earlier this month.
The company says its shutdown decision stands. The company says the union representing workers rejected concessions at the plant, which it says isn't globally competitive.
The Associated Press contributed to this story.