Washington State Patrol Trooper Chris Webb said the crash happened about 7:45 a.m. as the truck was getting onto the freeway from the State Route 599 on-ramp.
The truck, which was loaded with paper products, flipped onto its side and knocked over a light pole, blocking two lanes of southbound traffic and the on-ramp.
No one was hurt, and it was not clear exactly what caused the crash.
A large tow truck was called in to get the semi and trailer back on its wheels. All I-5 lanes and the on-ramp were reopened by 10:40 a.m.