Western Province have agreed to the early release of their 27-year-old utility back.
Although some paperwork is still to be completed on a proposed move to Vicarage Road, Saracens are delighted with progress made.
Saracens rugby director Alan Gaffney said: "It has not really been that much of a secret that we have been trying to sign Brent for a number of weeks, and we are delighted that he will be a Saracens player.
"We welcome the news that Western Province have agreed to release him early from his contract, and that has cleared the way for him to join us for the start of the season.
"There is still the odd bit of paperwork to complete, which should be resolved shortly, and I know that Brent is looking forward to competing in the Guinness Premiership and Heineken Cup this year.
"He is a quality player who has the ability to change games in an instant.
"He can play in a number of different positions including full-back, wing and fly-half, and he has pace to burn, which really makes him an exciting player."
Russell has won 18 caps for South Africa, having made his debut against Wales in 2002.