Jack Ross says he is taking charge of an "incredible football club" after being confirmed as Sunderland manager on a two-year contract.
Ross, 41, won the Scottish Championship title with St Mirren in 2017-18.
He has succeeded ex-Wales boss Chris Coleman at Sunderland, who will play in League One next season.
Successive relegations in the past two seasons have meant the club dropped from the Premier League to England's third tier.
"If you look the history, the facilities and most important of all, the fan base, you can see what this club can be," said Ross.
"To be part of the team to help realise that potential is something that fills me with excitement."
Ross 'has belief in Sunderland vision'
Ross' arrival is the first significant appointment made by new owner Stewart Donald, following his takeover of the club on Monday.
Donald added: "Jack is excited to be a part of the future we are building here.
"The fact he turned down lucrative offers from elsewhere, including clubs currently in a higher division than ourselves, is testament to his commitment and desire to succeed at this club and his belief in our vision for the future."
Ross will be joined at Sunderland by his assistant James Fowler.
St Mirren thanked Ross for "ensuring St Mirren were protected at all times" and "commend his commitment to the job throughout periods of speculation", while also wishing him "every success in his new challenge".
A club statement added that the search for Ross' replacement is now under way.
Who is Jack Ross?
Falkirk-born Ross has enjoyed a mostly upward curve in management since taking his first job at Alloa Athletic, and continued that following his move to St Mirren.
Promotion in his first full season in charge at St Mirren Park, earned him the PFA Scotland Manager of the Year award on the back of a campaign in which his side won 23 games to finish 12 points clear of their nearest rivals.
Ross had only arrived at the club the previous October, but led the Buddies from the bottom of the league to a seventh-placed finish and safety, having spent all but four weeks of the campaign in the bottom two.
He had left Alloa in second place in Scottish League One when moving on to the Saints, despite enduring relegation in his first campaign.
Before retirement, the defender enjoyed a solid playing career predominantly in Scotland with clubs including Falkirk, the Buddies and Hamilton Academicals, as well as a short-lived spell at Hartlepool United.