Walker, 30, has hit .272 with 93 home runs and 418 RBIs through seven MLB seasons, all spent with the Pirates. In 2015, Walker hit .269 with 16 home runs and 71 RBIs.
Walker was selected by Pittsburgh in the first round of the 2004 draft. He won the Silver Slugger Award in 2014.
Niese, 29, went 9–10 with a 4.13 ERA in 2015 for the National League champion Mets. The lefthander previously spent each of his eight MLB seasons in New York. He has a 61–61 career record with a 3.91 ERA.
The deal is pending medical review.