The catwalk is the elevated walkway behind the railing next to the lanes of traffic.
Police were called at around 2:30 p.m. after drivers saw a man on the catwalk walking towards Manhattan.
When Port Authority Police confronted him, he was not cooperative and combative. Police eventually took him into custody. He was found with a knife and drugs on him.
He will be charged with trespassing, weapons possession and drug possession.
Port Authority Police are investigating how he got into the tunnel and what he was doing there.
The south tube, which is the dedicated inbound tube, was closed for about 20 minutes. It reopened at 2:51 p.m.