Goalkeeper Westwood is moving to the Stadium of Light after rejecting a new deal from his previous club Coventry City.
The 26-year-old joined the Sky Blues from Carlisle United in 2008 for a fee in the region of £500,000, making 138 appearances for the club.
Swedish winger Larsson joins from Bimingham City, who released him following their relegation last season.
Current Black Cats manager Steve Bruce had previously signed the player, 26, to City when he managed there in 2007. Larsson went on to make 170 appearances.