An eyewitness saw police officers outside the 8th floor flat unit at Block 330 Clementi Avenue 2 at about 3.30pm.
SCDF officers were seen rappelling down to the kitchen window of the unit, ready to enter if necessary.
A male voice was heard shouting inside unit, followed by silence.
Police officers then broke down the main door of the unit and found the man covered in blood.
He had stabbed himself in the chest and slit his wrists.
Paramedics rushed him to hospital and he was conscious upon arrival.
The case has been classified as an attempted suicide.
Police investigations are ongoing.