In a video statement Mr Trump said he ordered a "targeted military strike" against the airbase from which the attack was believed to have been launched.
U-S officials say the targets included an airstrip, aircraft and fuel stations near the city of Homs in western Syria.
It has been confirmed that 59 Tomahawk cruise missiles were launched from U-S Navy destroyers in the Mediterranean Sea.
Mr Trump says he was compelled to act.
Donald Trump says it is in the vital national security interest of the U-S to prevent the spread and use of chemical weapons.
He has called on other countries to join the U-S in seeking to end the bloodshed and slaughter in Syria.

