The 28-year-old Armas was 9-12 with a 5.03 ERA in 30 starts with the Washington Nationals last season. He made his major league debut with Montreal in 1999 and has a 48-60 record and a 4.45 ERA in 151 career starts.
"He fits in with what we're looking for. He gives us depth and experience," Littlefield said. "He's a talented young guy who's battled back from some injuries. He was a big-time prospect who got to the big leagues quick."
Armas was 12-12 with a 4.44 ERA in 29 starts for Montreal in 2002. He was limited to five starts a season later after developing a shoulder injury, but has made 65 starts over the last three seasons.
The Pirates retained Shawn Chacon with a one-year deal worth $3.8 million last month. And their plan, is to use spring training to determine which of those two -- and, possibly, Shane Youman or Marty McLeary -- will start and which will work in long relief.
In addition to including Youman and McLeary in the mix, Littlefield indicated that none of the four young starters -- Ian Snell, Zach Duke, Paul Maholm and Tom Gorzelanny -- should take their spots for granted.
The Pirates signed Armas after losing out in the bidding for Jeff Weaver, Tomo Ohka and Brian Lawrence over the course of the past few weeks.
"I don't want to say (the Pirates' off-season moves) are complete," Littlefield said. "There are still a few guys out there who might fit our plans."
Littlefield would like to add some right-handed depth to the bullpen. Also, he will continue to search for a righty-hitting backup outfielder and a lefty backup infielder.
Armas' father, All-Star outfielder Tony Armas Sr., played 14 seasons in the majors with the Pirates (1976), Athletics (1977-83), Red Sox (1984-86) and Angels (987-89).
To make room on their 40-man roster for Armas Jr., the Pirates designated right-handed pitcher Franquelis Osoria for assignment.