The man, identified by another protester as Shaun O'Kelly, climbed up during the night and was still on the wall as of noon.
Five people were arrested in the plaza for illegal camping around 3:20 a.m. today, San Jose police Officer Jose Garcia said. One was cited and released at the scene, and the other four were booked into Santa Clara County Jail, he said.
Occupy San Jose protester Peter Brown said that at least three tents were pitched outside of City Hall when he left on Sunday afternoon. He said that when he returned early this morning, O'Kelly was already on the wall.
Garcia described the wall as "decorative" and estimated it is at least two stories high.
"It's pretty tall," he said.
Garcia said it could be dangerous to try to remove the man, so police are going to let him be for now.
"We're just going to wait him out until he comes down," Garcia said.
Early this morning, the man was standing on the wall holding a sign that read, "Stand up for what you believe in." He would occasionally flip the sign to show the back, which read Knowmore.org
The man had also hung additional signs on the wall that appeared to be made out of cardboard, listing numbers of police officers and protesters.