The 26-year-old Finnish center was drafted by the Blues in the third round (No. 65) of the 2008 NHL Draft.
Lehtera has played the past three seasons with Novosibirsk Sibir of the Kontinental Hockey League. In 48 games in the KHL last season, he scored 12 goals and 44 points, which led the team.
Lehtera also won a bronze medal with Finland at the 2014 Sochi Olympics, scoring one goal and four points in six games.
"We are excited to bring Jori to the NHL during the prime of his career," Blues general manager Doug Armstrong said. "His top-level skill has not only allowed him to be one of the KHL's top point producers over the past few seasons, but enabled him to excel against the world's best players at both the 2014 Olympics and numerous other World Championships."