Fields, 28, missed most of last season recovering from right hip surgery, which he underwent in late April. Fields came off the disabled list on Sept. 1 and finished the season by appearing in 13 games with the Royals. He went 15-for-49 with three homers and six RBIs during that brief stint.
Fields was eligible for arbitration this offseason but was not tendered a contract by Kansas City, which made him a free agent.
A former first-round pick in the 2004 First-Year Player Draft, Fields has appeared in the Majors in each of the last five seasons. He broke in with the White Sox in '06, played in a career-high 100 games in '07 and remained with Chicago until being traded to the Royals in November 2009.
In 796 career at-bats, Fields has hit .234 with 25 doubles, 34 homers and 107 RBIs. Twenty-three of those home runs came in '07, when the right-handed-hitting Fields drove in a career-best 67 runs.
Fields came up through the White Sox system as a third baseman and has played mostly at third in the Majors. He has also played 22 games in left field and 17 at first base.
Fields will compete to make the Pirates' roster as a utility player -- in much the same way that another former top prospect, Andy Marte, will. The Pirates need someone who can back up Pedro Alvarez at third -- and both Fields and Marte, who signed a Minor League deal with the club earlier this month, could fill that role.