A three-time champion in Rome, Swiatek took little more than an hour to take care of fifth seed Pegula on centre court, in a show of force on her preferred surface not seen since she last won the French Open two years ago.
Swiatek has set up a clash with either second seed Elena Rybakina or Elina Svitolina in the last four.
A potential decider with reigning Roland Garros champion Coco Gauff awaits for the winner of that last-four match-up in Saturday's final.
Pole Swiatek hasn't won a clay-court tournament since claiming the most recent of her four Roland Garros titles, with personal problems a factor in her poor performances.