Chesney will take on Leicester in his final Saracens appearance
Kris Chesney will play his last game for Saracens this weekend after agreeing to join French side Toulon.
The 35-year-old has made 337 appearances for the Vicarage Road team.
Meanwhile, Saracens have made five signings - Italian forward Carlos Nieto from Gloucester and Fifi wing Michael Tagicakibau from London Welsh.
Three English signings, Treviso full-back Michael Horak, London Irish prop Richard Skuse and Exeter scrum-half Kevin Barrett, also join the squad.
Toulon, backed by big-spending president Mourad Boudjellal, have landed some of the highest-profile imports into the French Top 14.
Chesney's new team-mates could include former All Blacks Jerry Collins and Tana Umaga, Springbok Joe van Niekerk and rugby league convert Sonny Bill Williams, all of whom are current members of the squad.
"It was not an easy decision, but I did want a new challenge and made the decision to join Toulon at Christmas," said Chesney.
"I wanted to go abroad because I couldn't picture myself playing for any other club in England other than Saracens."
Despite Chesney's exit, Saracens' incoming director of rugby, Brendan Venter, says the additions to the Saracens set-up highlight a belief in domestic talent.
"As a club, we are determined to develop English players and deliver them into the England team," he said.
All three of the English signings have played for London Irish at points in their careers, with 31-year-old Horak earning his solitary England cap against Argentina in 2002 while playing his club rugby for the Exiles.
"We have a fantastic number of exciting and hugely talented youngsters in the backline, and Michael will provide a much-needed experienced head at the back," said Venter.
Horak, Skuse and Barrett will all be playing for Saracens next season
"He has a strong left foot, which is always useful in a full-back, but perhaps most importantly he is player of immense character."
Now 28, Skuse has represented England at under-21 level and has spent five seasons with London Irish since signing from Bristol.
"Richard is an outstanding player, who has been prevented by injury from fulfilling his potential at London Irish. We believe we can accelerate his development at Saracens, and help him win international honours," Venter added.
Barrett, 28, has become a regular with the England Sevens side and helped the side to their first ever tournament win in the IRB Sevens World Series in Wellington in February.
"Kevin is a livewire scrum-half, and it is not surprising that he has made such an impact with Exeter and in the England Sevens squad," continued Venter.
Tight-head prop Nieto, 32, joins from Gloucester where he made 64 appearances in the last three seasons.
He joins fellow Azzurri front rows Matias Aguero and Fabio Ongaro at Vicarage Road.
"We looked long and hard for the right man with right work ethic," said Venter.
"In Carlos, we have found a player who is widely respected in the game and, importantly, who is tried and tested in the Guinness Premiership."
Saracens are expected to announce another six signings, consisting mainly of South Africans and Italians, this week as part of a dramatic overhaul of the club's playing staff.
Fifteen members of the current squad were informed in February that they will be released at the end of the season.
A South African consortium led by businessman Johann Rupert has invested heavily in Saracens and chief executive Edward Griffiths has outlined a new push to recruit supporters from London's large South African expatriate community.