The Black Cats are also poised to make a third summer signing, with South Korean striker Ji Dong-won due to fly into Wearside to complete a medical after Chunnam Dragons agreed a fee that could rise to around £2m for his services.
After weeks of patient groundwork, the last 48 hours have seen Steve Bruce finally make a series of much-needed breakthroughs as he attempts to rebuild a squad that has lost four loan players and Jordan Henderson since the end of last season.
Larsson was on Wearside yesterday to tie up the loose ends of his Sunderland contract, having spurned a last-gasp offer from Aston Villa boss Alex McLeish this week.
The Sweden international, who was also offered moves to Stoke City and Blackburn Rovers, boasts considerable Premier League experience, having made more than 150 league starts in his four seasons with Birmingham City.
He can play on either flank, but tended to line up on the right-hand side for Blues, making him a ready-made replacement for Henderson, who completed a £19.5m move to Liverpool earlier this month.
Larsson is due to become a free agent at the start of next month, which is why Sunderland will not be in a position to officially confirm his arrival for another eight days.
The same applies to Westwood, who has also agreed the terms of his own move to the Stadium of Light.
The 26-year-old has signed a four-year contract despite competing interest from a number of clubs including Leeds United and Wolves, and is expected to start pre-season as Sunderland's number one goalkeeper ahead of both Simon Mignolet and Craig Gordon.
The Republic of Ireland international started his career as a trainee with Manchester City, and made more than 140 appearances for Carlisle United before joining Coventry City in 2008.
He confirmed he would be leaving the Ricoh Arena yesterday, with Coventry boss Andy Thorn saying: Kieren has written to the club confirming that he will not be signing a new deal. He has been an outstanding player for this club and I would personally like to wish him all the best for the future.
Like Larsson, Westwood will not be officially confirmed as a Sunderland player until July 1, and by that stage Ji's transfer from South Korea should be completed.
The 20-year-old striker, who impressed as he scored four goals in January's Asia Cup, has agreed a three-year deal with the Black Cats, who have fought off late competition from Dutch side PSV Eindhoven to secure his signature.
Ji Dong-won is joining Sunderland on a three-year contract, said a statement on Chunnam's website.
The terms negotiated by the two clubs regarding transfer fee and salary will not be released publicly.
Ji, who scored 11 goals in 39 appearances for Chunnam, is still to complete a medical, but is looking forward to playing in the Premier League.
I had the impression that Sunderland really wanted me, said the striker. I know there will be stiff competition for playing time, and my goal is to become the player that the team wants me to be in six months.
I am also looking forward to playing against (Park) Ji-sung and (Lee) Chung-yong, whom I both admire.
In total, Bruce is hoping to make up to eight signings this summer, and having tabled a number of bids in the immediate aftermath of Henderson's departure, the boss remains hopeful of luring Darron Gibson, John O'Shea, Wes Brown and David Ngog to the Stadium of Light.
Manchester United have accepted a combined £12m offer for Gibson, O'Shea and Brown, but talks with the players and their representatives remain ongoing.
Gibson has been the most receptive of the trio, with O'Shea and Brown more reluctant to leave Old Trafford despite Sir Alex Ferguson's willingness to part company with them.
The financial intricacies of the trio's departure from Manchester United are still to be agreed, but Sunderland remain hopeful all three can be persuaded to move to the North-East.
There have also been talks with Ngog and his representatives, but the terms of the Frenchman's proposed departure from Anfield are understood to be complicating any deal.