The incident unfolded at about 7:30 p.m. Sunday in the 2700 block of Elliott Avenue when the apartment resident spotted the suspect on his balcony and confronted him, police said.
The suspect clambered down a metal trellis then ran away towards Pier 69. The victim followed the suspect away from the building towards the waterfront.
The suspect then scaled a fence and yelled at the victim that he had a gun and entered Pier 69. The victim stopped chasing the suspect at that point and called 911.
Police arrived and surrounded the buildings on Pier 69 and requested Harbor Patrol units to search the water in the surrounding area as well as underneath the pier, said Patrick Michaud of the Seattle police.
As officers searched the area, the Victoria Clipper III was forced to remain away from the dock out as a precaution to protect the safety of the passengers who were on board. Officers with the Seattle police and Port of Seattle conducted a thorough search but did not locate the suspect.
The suspect is described as a white male, in his mid to late twenties wearing a white T-shirt, dark jeans and had a dark sweatshirt tied around his waist.
Anyone who sees the suspect is urged to call 911 and use caution as he may be armed.