The transaction happened on Thursday at the Major League Baseball winter meetings for the 24-year-old right-hander, who pitched for Tulsa, a Class AA farm team for the Colorado Rockies.
Under Rule 5 the Mets must put Register on their 25-man roster or offer him back to the Rockies for a $25,000 fee. The Mets have to pay the Rockies $50,000 for the draft choice after the Rockies did not protect him on their 40-man roster.
He posted a 1-3 record last season with 37 saves and a 4.03 earned run average. He allowed 63 hits in 58 innings and made a total of 61 appearances. He struck out 48 batters and walked 16.
Prior to going to the bullpen, he posted a three-year pro record of 19-28 as a starter with a 4.73 earned run average.