Kock, 26, has won two caps for the Springboks and joins from Super Rugby side Stormers on a two-year deal.
Maitland moves to Allianz Park from relegated London Irish and has agreed a "long-term contract" with Sarries.
The 27-year-old, who can also play at full-back, has featured 21 times for Scotland since his debut in 2013.
"We identified Vincent as a player who could make a significant contribution to the squad at tight-head and he is a player with huge potential," Saracens director of rugby Mark McCall said.
"Sean is a highly experienced and proven international three-quarter, and we believe that he will flourish here.
"Both players are exactly the kind of characters we think will thrive in our environment, and we are confident they will become key players."
Premiership and European Champions Cup winners Saracens have signed South Africa prop Vincent Koch and Scotland winger Sean Maitland.