The checking-line forward signed a US$3-million, two-year deal after free agency opened on Sunday.
McClement is a seven-year NHL veteran who spent last season with the Colorado Avalanche, where he led the team in short-handed time on ice.
Toronto has been ranked no higher than 24th in the NHL in penalty killing since the 2004-05 lockout.
In 553 career games with St. Louis and Colorado, the 29-year-old McClement has 63 goals and 177 points.
Also Sunday, the Leafs signed forward Matt Frattin to a two-year contract.