The Black Cats have brought the defender to the Stadium of Light after leaving Coventry at the end of last season.
The 24-year-old defender, who has signed a two-year deal, made 208 appearances for the Sky Blues, coming through the ranks at the West Midlands club before making his debut in the Championship in 2011.
As mentioned on the Sunderland website, Willis captained the club to the EFL Trophy in 2017 before helping them earn promotion via the League Two playoffs the following season.
Here is what some Black Cats fans on the social media platform made of the signing:
Great signing, looked solid against us last season. Welcome to Sunderland ⚪️— kyle (@kylemurg) 13 July 2019
Maybe should have give the lad 3 year deal coz a think he will turn out to be a very good player for us— newby08 (@newby08) 13 July 2019
Welcome to Sunderland! Ha’way the lads!!! ⚪️⚪️— SAFC North Yorkshire (@SAFC_NYorkshire) 13 July 2019
Great signing! Dross out, quality in. Keep it up⚪️— Neil Foster (@drydenfoster) 13 July 2019
Looks a unit. What we need!— MR (@Discw0rld) 13 July 2019
What a signing! To turn down multiple championship clubs to come to us shows everyone that he wants to be a part of our journey. These are the type of players we need. It's nice to see how many Coventry fans are bitter over this shows you how good this lad really is.#SAFC— Kieron Weatherall (@KieWeatherall_) 13 July 2019
Good bit of business. Good luck m8. Good to see the true coventry fans wishing him well.nice one.— Steve (@JLwest40) 13 July 2019
Really pleased with that signing ⚪⚪— Paul Grafton (@PGrafton78) 13 July 2019
Willis is Sunderland's third signing this summer following Lee Burge's arrival at the Stadium of Light - also following his release by Coventry - and Conor McLaughlin.
The Black Cats finished in the top six last season and made it to the playoff final, only to suffer a heartbreaking late loss to Charlton Athletic at Wembley.
Have something to tell us about this article?