Robertson will be paid $5.5 million on a pro-rated $16 million deal, with actual cost for the Phillies being $12 million.
Pro-rated contracts in baseball mean that a free agent's (Robertson) signed salary ($16 million) is adjusted ($5.5 million) based on the number of games already played.
He will look to help a Phillies bullpen that ranks 23rd in the league with a 4.36 ERA.
Robertson previously played for the Phillies on two separate occasions - first in 2019, when he pitched in seven games before undergoing season-ending elbow surgery, and again in 2022 after being traded from the Chicago Cubs.
Despite having just one All-Star appearance in what is now going to be 17 years in the MLB, Robertson has consistently been one of the best relievers in the league.
He posted a 3.00 earned run average in 72 innings last season for the Texas Rangers while striking out 99 and walking 27, making his strikeouts per nine innings rate (12.4) the highest it has been since 2017 (12.9).
Robertson has posted a stellar career 2.91 ERA in 876.2 innings with 1,154 strikeouts to just 357 walks.
After struggling in 25 games his rookie year (2008) with a 5.34 ERA, Robertson went on to be part of the 2009 World Series Yankees team the following year and never finished a full season with an ERA above 4.00.
Robertson has played for eight teams throughout his career, which includes the Yankees (nine years), Chicago White Sox (3), Phillies (3), Rangers, New York Mets, Miami Marlins, Cubs, and Rays.