Sugawara agreed a four-year contract with the newly-promoted Premier League club.
The 24-year-old right-back is Southampton manager Russell Martin's seventh signing since last season's play-off final victory over Leeds at Wembley.
Sugawara played 198 games for Alkmaar in all competitions after joining from Japanese club Nagoya Grampus in 2018.
He has been capped 13 times by Japan and made his international debut in 2020.
"To come to Southampton and to play in the Premier League is one of my biggest dreams since I was young, so I am really excited to be here," Sugawara said.
"I felt Southampton is a really good club. When Southampton gave me an offer, I said yes after one second."
Martin said he had been tracking Sugawara for "a long time" before finally getting his man.
"He arrives at a good age with a lot of games already under his belt at a high level, including a wealth of European experience," he said.
"He's a talented footballer with a great mentality, and a character we think will be really popular with his team-mates. You can sense his enthusiasm and determination to do well."
