The Egypt international joins City for a fee in the region of €70m. He has signed a deal with City to 2029.
“This is a day I will never forget,” Marmoush said.
“To sign for Manchester City – one of the best teams in the world – is an amazing feeling. I am delighted, my family are so proud, and we are all very happy to be here in Manchester.
“With Pep, his technical staff and the world-class facilities here, players have everything they need to improve. That was really enticing for me when I had the chance to come here.
“And I cannot deny I also want to win trophies. City have been the most successful club in England for many, many years, so I know I am joining a winning environment and winning culture. I want to learn from the staff and my teammates, and I want to become a valued member of this winning team.
“I really am looking forward to getting going, meeting the other players and showing the Manchester City fans what I can do.”
City sports chief Txiki Begiristain also said: “Omar is an accomplished and exciting forward, and I’m delighted he’s joining us.
“He’s had an outstanding season, and every time we have watched him, he has influenced matches.
“He has all the attributes a top-class attacker requires. He has outstanding pace and awareness, and he is exceptional in front of goal. He can also play a number of different positions, which is a really valuable asset.
“I also have no doubt that working with Pep and the technical staff here at City will see him further develop his superb attacking talent.”