Swedish Larsson joins from Birmingham City after being released due to the club’s relegation in May. He underwent a medical for the Black Cats yesterday after his former Blues boss Alex McLeish had made a last-minute attempt to sign him for Aston Villa.
Westwood is entering into a four-year contract after rejecting a new deal from his previous club Coventry City.
Both transfers will be made official on July 1.
Meanwhile, Ji Dong-Won is expected to join the Stadium of Light from South Korea’s Chunnam Dragons after he has undergone a medical.