The Tay Son reinforcements led by Nguyễn Huệ march south from Quy Nhon and arrive in Cochin China territory soon after.

Hue avoids a direct attack on a strong Siamese force at Sa Dec and tries to set up a trap.

Nguyễn Huệ, anticipating a move from the Siamese, has secretly positioned his infantry and artillery along the Mekong River (Rạch Gầm-Xoài Mút area of present-day Tiền Giang province), and on some islands in the middle, facing other troops on the northern banks with naval reinforcements on both sides of the infantry positions.

On the morning of January 20, 1785, Nguyễn Huệ sends a small naval force, under a banner of truce, to offer to parley with the Siamese.

After so many victories, the Siamese army and naval forces are overconfident.

They go to the parley unaware of the trap.

Nguyễn Huệ's ships dash into the unprepared Siamese troops, preventing their advance or retreat.

In the meanwhile, Tay Son artillery open fire.

The battle ends with a near annihilation of the Siamese force, at least according to Vietnamese sources reporting that all the ships of the Siamese navy were destroyed and only two thousand to three thousand of the original expedition survived to escape back across the river into Siam.

Nguyễn Ánh and his family members escape and will later go to Siam.