Meghan Markle's Half-Brother Thomas Arrested for DUI

Meghan Markle's half-brother, Thomas, was arrested Friday for driving under the influence of alcohol ... TMZ has learned.

The 52-year-old was driving by Oregon State Police when police stopped him at 1:33 AM and took him into custody for impaired driving.

Cops performed a field sobriety test and they say his impairment was obvious. They say he was visibly intoxicated and blew a .11.

Thomas -- the son of Meghan's father, Thomas Sr. -- was taken to the Grants Pass Sobering Center. His Chevy Silverado was impounded.

Thomas is estranged from Meghan. He sent a letter to Prince Harry in April in which he said, "It's not too late. Meghan Markle is obviously not the right woman for you."

Thomas' dad tells TMZ, "My son has a serious problem and he needs help, and I hope he gets it." Thomas Jr.'s sister, Samantha, tells TMZ, "His drinking is not related to Meghan. He always drank."