A man wearing a samurai outfit jumped the fare gate at Paya Lebar MRT station and took a samurai sword on board a train, SMRT said in a statement on its Facebook page on Monday.
Its staff alerted the police and the man was later arrested near Bugis MRT station.
The statement added: "Passenger safety is our priority and we do not tolerate abhorrent behaviour nor allow offensive weapons to be brought on our trains.
"Passengers who witness such acts should activate the emergency communication button to communicate with the train captain, who will in turn report the situation to the operations control centre. Station staff at the nearest station will render immediate assistance and will alert the police and security staff."
When The New Paper contacted SMRT, the public transport operator said no further information would be provided.
The police said they received a call at about 12.46pm requesting for assistance at Paya Lebar MRT station.
When their officers boarded the train travelling towards City Hall, they kept the suspect apart from other commuters so as not to aggravate the situation. He alighted at City Hall station, according to the police.
"At Bugis, officers advised the suspect to surrender his weapon. The suspect complied and was subsequently arrested for being in possession of a schedule weapon under Section 7(1)(a) of the Corrosive and Explosive Substances and Offensive Weapons Act, Chapter 65," the statement added.