The Hearts centre-back agreed a pre-contract which was announced last week.
It has been reported that Rangers will try and do a deal to bring the Scotland international to Ibrox in January.
Hearts noted the player would remain part of the first-team plans “unless a significant and satisfactory offer is made” this month.
Speaking on Souttar's arrival ahead of the Aberdeen game, Rangers boss Giovanni Van Bronckhorst said: “I am happy that John (Souttar) will be joining us.
"A top Scottish international and I look forward to him joining us next season.”
For Rangers, both Connor Goldson and Leon Balogun’s contracts are up at the end of the season.
Regarding Goldson, who has played the most amount of minutes since joining the club back in 2019, there has been little suggestion he will put pen to paper on an extension.
Meanwhile, Nikola Katic is due to return from his loan spell at Hadjuk Split.
The centre-back is getting much-needed game time in Croatia after a long injury lay-off which saw him miss the entirety of the 2020/21 campaign. The 25-year-old has played ten times with the league set to resume at the end of the month.
“Katic is playing a lot which is the main thing when on-loan,” Van Bronckhorst said.
“Hopefully, he will continue to develop himself as a player and we can then discuss in the summer.”