Singapore is surrounded by water and can therefore be affected by a tsunami. However, the megathrust tsunamis like the one that hit the Indian Ocean in 2004 will not affect Singapore: the Indonesian archipelago acts as a barrier that protects the city from such events.
Nonetheless, tsunamis generated by submarine landslides or eruptions in the Sunda shelf can potentially affect Singapore. That is why scientists investigate plausible past tsunami deposits on the shores of the South China Sea and the Java Sea.