The Phillies, according to Todd Zolecki of MLB.com, are making that gesture as part of a tribute to Halladay, who died when his plane crashed into the Gulf of Mexico on Nov. 7. He was 40.
With the Phillies, Halladay threw a perfect game in 2010 against the Marlins. Less than five months later, he pitched the second no-hitter in postseason history when he defeated the Cincinnati Reds in Game 1 of the National League Division Series.
Halladay also served as a mental skills coach for the team this past spring in Clearwater, Fla.
Catcher Andrew Knapp wore No. 34 for the Phillies last season, but reportedly expressed reservations about taking Halladay's number. He switched to his college number (No. 15) after Halladay died.
It is not known if the Phillies will retire the jersey number for longer than the 2018 season.
Halladay's life will also be celebrated by the Toronto Blue Jays next season, beginning with a ceremony prior to their home opener on March 29.
Halladay went 203-105 with a 3.38 ERA during a 16-year career. He pitched for the Blue Jays from 1998-2009 and Phillies (2010-13) before retiring.
An eight-time All-Star, Halladay won the American League Cy Young in 2003 when he went 22-7 with a 3.25 ERA for the Blue Jays. He won the NL Cy Young in 2010 when he went 21-10 with a 2.44 ERA in his first season with the Phillies.
He was a combined 40-16 in his first two seasons in Philadelphia and was 55-29 with a 3.25 ERA in his four campaigns with the Phillies.
Halladay was 148-76 with a 3.43 ERA during his tenure with the Blue Jays, winning 16 or more games on six occasions with the club.