The Russian will be retained after strong recent performances, with his season being blighted by poor form prior to the summer break.
"It makes a change to announce our driver line-up relatively early," Tost said. "There are so many new elements coming to Formula One in general and to our team specifically, in terms of the change of power unit supplier, that having the same two drivers gives us stability and a benchmark to work from."
"For Carlos, it will be his third year with us, which speaks volumes when it comes to how highly we rate him," he added, speaking of Carlos Sainz who had previously been linked with a move to Renault.
"In recent races, it has been clear that Daniil is back on top form," he said. "I always told him that his future with us was in his hands and he has stepped up to the mark and delivered the sort of performances that have ensured his 2017 seat in the STR 12."
"We now have a very talented and strong driver pairing to tackle a season in which we expect to be very competitive."
Bahrain International Circuit - Winter testing