The Northampton Saints wing will join up with the Aviva Premiership champions at the start of the 2012-13 season, therefore continuing to play for the Saints for the rest of this term.
A spokesman for Saracens said: "We are obviously especially delighted but, for now, Chris remains a Northampton player. Out of respect for Saints, Saracens will not make any further comment on the matter."
Ashton wrote on Twitter: "I've decided to join Saracens from next season... until then it's all about Saints."
The 24-year-old has 88 tries to his name since joining Northampton in 2007.