He was having his breakfast at a hawker centre in Clementi last Wednesday, when he suddenly heard a loud noise.
When full-time national serviceman Koh Zhi Hao, 22, turned around, he saw a man in his 40s shaking uncontrollably on the floor, with his eyes shut and saliva flowing out of his mouth.
The man was believed to have suffered a seizure.
Third Sergeant Koh, who currently serves on the Singapore Navy's patrol vessel RSS Daring of 182 Squadron, saw there were about 15 people in the hawker centre but sensed that no one intended to step forward to help.
He immediately rushed to the man and called 995.
Following the advice of a medic over the telephone, 3SG Koh tilted the man on his side to let him breathe and continued supporting him alone till the ambulance arrived five minutes later.
The Singapore Civil Defence Force confirmed that it responded to a call for ambulance assistance in Clementi West Street 2, and said a man was taken to the National University Hospital in a conscious state.
Disappointed by the lack of response from onlookers, 3SG Koh decided to share his story on online forum Reddit, under the title "Your hesitation can kill/save someone", to encourage people to provide assistance in such cases.
His actions were lauded on the forum, with the post gaining more than 100 comments which praised his quick thinking.
He said: "I was quite appalled by the lack of willingness to help and, through my Reddit post, I wanted to inspire people to be more proactive and to adopt a more inclusive thinking.
"I did not expect such a (big) response but most of the posts are by responders... so I guess the message resonates with them and they know that this is a message that has to get out."
Among those who praised him was Reddit user Darndc, who said: "As a healthcare professional, kudos and big thanks to you. What you did, regardless of whether you know what to do or not, is already half the work when responding to incidents like this."