The young left-sided player is currently with the Switzerland squad having played every game in their run to the semi-finals of the European U19 Championship.
"Reto Ziegler is a very young player who has played at the highest level in Switzerland and has been in the Grassshopper first team for one -and-a-half seasons despite being only 18 years old," said our Sporting Director Frank Arnesen.
"A lot of big European clubs tried to sign him. I am delighted that we could.
"He loves English football and believes that Tottenham is the right club to bring him along further.
"He is a left-back with the capacity to play midfield. He holds a lot of promise and I look forward to him joining us on the 1st January next year."
The deal remains subject to formalities being completed.
The club is delighted to announce that we have reached agreement with Grasshoppers Zurich to sign Reto Ziegler at the end of the Swiss season in January.