Part of the City side that won the Premier League 2 title last term, the 21-year-old defensive midfielder has been allowed to continue his education in the Championship at Oakwell.
The former French youth international, named in the 2017 European U17 Championships Team of the Tournament, Gomes joined City’s Elite Development Squad from Paris Saint-Germain three years ago.
Gomes, capped by France up to under-20 level, made his City debut in the 2-0 win over Chelsea in the 2018 Community Shield win at Wembley.
The midfield enforcer made one further appearance for City in the EFL Cup in that 2018-19 season and featured twice in the Football League Trophy.
Last season, he made his FA Cup debut as a substitute for Rodri in a 3–1 win at Swansea City in February, while also appearing for City under-21s side on a couple of occasions in the EFL Trophy.
The Frenchman came off the bench in City's 4-0 victory over Barnsley in a pre-season friendly in the summer.
On joining the club, Gomes said: “I’m excited because I want to play men’s football and I think, for me, it’s a good place to play and get experience.
“I know the Championship is a good place in England and I’m excited to start playing with the guys here.
“I’ll try to do my best to help the team move forward, maybe more than last season and ready to help the team go higher in the table.”
Barnsley FC co-chairman Paul Conway said: “It was important for the club to get its first four signings completed early, allowing us time to complement the squad later in the window.
"Now, we are pleased to welcome Claudio to Barnsley. Having associations with clubs like PSG and Manchester City, Claudio’s football education is strong and it’s clear that he has a bright future ahead of him.
“We look forward to integrating Claudio into the first team and providing Markus with another quality option.”