Officials reported four of the fatalities in Houston, where storms poured 11 inches of rain overnight and inundated sections of America's fourth-largest city.
Firefighters conducted over 500 water rescues Tuesday and drivers abandoned at least 2,500 cars to the floodwaters.
Most of the gaping waterways fell back within their banks by the end of the day, but the flooding had shuttered several highways around the city for most of Tuesday. Officials scrambled to respond.
"Given the magnitude and how quickly it happened, in such a short period of time, I've never seen this before," said Rick Flanagan, Houston's emergency management coordinator.
The hunt continues for about a dozen other people missing, including eight from a vacation home swept away by the Blanco River in the hard-hit town of Wimberley in Central Texas early Sunday, according to New York Daily News.
The National Weather Service cautioned that more rainfall could be ahead for both states, with forecasters predicting a 40 percent chance of thunderstorms in northern Texas and areas of Oklahoma, Kansas, Missouri and Illinois on Wednesday. Houston itself faces a 20 to 40 percent chance of thunderstorms through the end of the week.