State troopers responded to the scene at about 12:30 a.m. after receiving reports of a crash involving a wrong-way driver.
Two people were in the car that was struck. One of them walked away with minor injuries, but the other was rushed to Harborview Medical Center with lacerations and at least one serious fracture.
A preliminary investigation found that the wrong-way driver, identified as Michael R. McDonald, 44, of Longbranch, most likely pulled onto I-5 at Marvin Road in a silver 2013 Hyundai and started heading north in the southbound lanes of the freeway.
He drove the wrong way for about three miles before striking the other car, a 1996 Toyota Tercel, near the Nisqually exit ramp.
When troopers arrived at the scene, they said McDonald looked lost and confused.
"Any time there is a wrong-way driver on the freeway, we automatically look for impairment," said Sgt. Robert Brazas of the State Patrol. It is not known yet whether drugs or alcohol were involved.
McDonald was placed under arrest and booked into the Thurston County jail for investigation of vehicular assault.
The crash shut down the southbound lanes of I-5 at the scene for about 3 1/2 hours. The highway has now reopened.