The Briton was in complete control throughout, breaking Shibahara's serve routinely on her way to a 6-1, 6-2 drubbing on the Louis Armstrong Stadium showcourt.
The win was Raducanu's first at the US Open since she made history four years ago, winning the championship as a little-known unseeded qualifier at the age of 18.
She was knocked out in the US Open first round in 2022, missed the 2023 tournament due to injury, and exited in the first round again in 2024.
"I'm very, very pleased to have won that match," Raducanu said in an on-court interview.
"It's my first win here since 2021 so it's extra special. The first round is always difficult, so I'm very happy to come through that.
"I'm very pleased with how I managed myself through that match."
The unseeded Raducanu will face Indonesia's Janice Tjen in the second round, after the qualifier stunned 24th seed Veronika Kudermetova later in the day.
