It happened on US-59 in a northbound lane between Reading Road and FM 762 Wednesday around 8:10 a.m.
Witnesses told authorities the man hit an animal they believed to be a dog. The driver stopped his vehicle in the road and was hit by a big rig. The driver of the 18-wheeler did stop.
A spokesperson for the Rosenberg Police Department called the death a horrible accident and doesn't believe any charges will be filed in the case.
Deputies were working to divert traffic to the feeder roads as Rosenberg police investigate.
Drivers are urged to find an alternate route because the northbound lanes of US-59 will be closed from FM-2218 to FM-762 for an extended period of time.