The move seems to be taking the spot of what was going to be a deal with Zobrist, who was said to be coming to New York this week before agreeing to join the Cubs.
The 30-year-old Walker will be a free agent after the 2016 campaign, and hit .269 with 16 home runs and 71 RBIs in 543 at-bats last season with the Pirates. He made $8 million in 2015 and is due an arbitration-eligible raise this year.
The Mets sent 29-year-old Niese the other way, who was 9-10 with a 4.13 ERA in 33 appearances last year. New York was looking to move him at the trade deadline this past season, but were unable to get a deal done and paid him $9 million.
The switch-hitting Walker showed more power the last two years in Pittsburgh, most of which came from the left side. He recorded a .456 slugging percentage from the left side of the plate, compared to just a .290 percentage from the right side.
The Mets would continue to bolster their infield later Wednesday night, agreeing to a two-year, $18.5 million deal with defensively-sound infielder Asdrubal Cabrera. The former Indian, known for his spectacular play in the field, had a .265 average with 15 home runs and 58 RBIs in 505 at-bats with the Tampa Bay Rays last season.
The Mets did a fantastic job of moving on in the Winter Meetings, as they will use both of their Wednesday acquisitions to fill a hole in a spot that was once Zobrist's to keep.