The 27-year-old joins the club from Southampton where he spent the 2024/25 season after joining the Saints from Arsenal last summer.
His time with the Gunners saw Ramsdale help his team to a second-place finish in the Premier League in the 2022/23 campaign, a season in which he made his senior England debut as well as being named in the squad for the FIFA World Cup in Qatar.
Prior to joining Arsenal, Ramsdale started his professional career with Sheffield United before moving to AFC Bournemouth where Eddie Howe handed him his top-flight debut in 2019. Across his Premier League career, the goalkeeper has made 183 appearances and kept 41 clean sheets.
On signing for Newcastle United, Ramsdale said: "It's great to be here. I've always loved coming here and I've seen how passionate and loud the fans can be. I've always loved playing here at St. James' Park - I've not always had the greatest of results, but when the fans get behind you it's some place.
"Coming to the stadium and seeing it empty today, it's quite surreal to think that this is going to be home for the foreseeable future. I've got the kit on, it’s got the nice Champions League badge on the sleeve and I'm delighted to be here.
"Working with the manager and his coaching staff again were a big draw for me. They've already had a great influence on my career and the gaffer turned me from a man into a professional footballer at Bournemouth. He really showed me the ropes so the manager and his coaching staff were a huge selling point for me to come here."
Head coach Eddie Howe said: "Aaron is an exceptional goalkeeper who adds further quality and international experience to our squad.
"Having worked with him before, I know what he can bring to us on and off the pitch, so we look forward to welcoming him into the group."