Billed as the strongest hurricane to hit the US in 12 years, Hurricane Harvey hit a stretch of coastline near the town of Rockport at around 10pm local time (4am UK time) as a category 4 storm with "life threatening" winds of 130 miles (215 km) an hour.
It is expected to dump over 3 feet (90 cm) of rain along the Texas coast and parts of Louisiana for several days.
Harvey has been compared to one of America's deadliest storms - Hurricane Katrina - which killed thousands in New Orleans in 2005. New Orleans does not lie in Harvey's direct path.
Donald Trump wrote on Twitter: "At the request of the Governor of Texas, I have signed the Disaster Proclamation, which unleashes the full force of government help!"
The President has been closely monitoring the approaching storm, and is currently in Camp David "closely watching the path and doings of Hurricane Harvey".
He earlier encouraged everyone in the path of the storm "to heed the advice and orders of their local and state officials".
The National Hurricane Center described the storm as "life-threatening, dangerous and catastrophic".
Sky News Correspondent Siobhan Robbins, who is in Texas, said: "Officials have warned that this storm could cut off power and water and homes could be uninhabitable for months.
"Thousands have already been evacuated, but the worry is that people have heard all this before and still aren't getting themselves to safety."
Seven counties in Texas have ordered mandatory evacuations, and tens of thousands of people left their homes ahead of the storm's arrival.
Mr Trump is expected to visit Texas early next week.
