The 20-year-old midfielder will link up with the Blades on a temporary basis for the 2022-23 campaign, after spending last season on loan with Hamburg and Cardiff City.
Doyle, who has become Sheffield United's first signing of the summer window, has now joined up with his new teammates in Portugal for their pre-season training camp.
Speaking to the club's official website on his arrival at Bramall Lane, Doyle said: "I'm excited to be here, I'm ready and looking forward to starting the season.
"I got the phone call that Sheffield United was interested and it is hard to say no to a club like this. I'm looking forward to meeting everyone, I know one or two already, but I'm focussed on getting down to work and preparing for the new season.
"I played for Cardiff at Bramall Lane last season and it wasn't easy. It was a tough day that I remember well, the fans and stadium were brilliant and I'm looking forward to playing there again."
Blades head coach Paul Heckingbottom added: "We are pleased to attract a player who has a lot of experience for his age. He's already had a couple of good loans, has been around the first team at Manchester City as well playing for England.
"We think he can add a lot of quality to our midfield, he can play in a couple of positions whether we play as a two or a three, and tactically he is very good, we are confident he will fit into our group very well, he's got the knowledge of how we want to play."
Doyle made only six Bundesliga 2 appearances for Hamburg last season but he enjoyed more minutes on the pitch with Cardiff in the second half of the 2021-22 campaign, scoring once and registering three assists in 18 Championship matches.
The Manchester-born midfielder has made a total of seven senior appearances for Pep Guardiola's Man City side and had previously been a regular for the club's youth teams, recording 13 goals and 21 assists in 84 games.
Doyle has also represented the England national team at every youth level from the Under-16s up to the Under-21s, including six appearances for the latter since the summer of 2021.