Ukraine’s Andrii Trusov topped the heats with a time of 27.44sec, followed by Egor Efrosinin (28.24) from the Russian Paralympic Committee and Ukrainian Yevhenii Bohodaiko (28.27).
The final will take place on Tuesday afternoon.
This is Toh’s second event at the Tokyo Paralympics.
He competed in the men’s S7 400m freestyle on Sunday, finishing seventh in the eight-swimmer final, and will take part in the men’s S7 50m butterfly on Friday.