"Philly raised little man will be! #BabyHarper," Harper wrote for the caption on two photos on the social media platform. Harper's first photo was a shot of himself standing with wife Kayla Harper with the Ben Franklin Bridge in the background.
The couple held a denim jacket reading "Baby Harper" with the No. 3 written on the back. The second photo Harper posted included a sonogram of the new addition.
The Harpers married in 2016 at the San Diego Mormon Temple.
The Phillies and Philadelphia Eagles congratulated Harper in the comment section, as did several MLB stars and celebrities, including: San Diego Padres first baseman Eric Hosmer, Washington Nationals slugger Matt Adams and country music star Jake Owen.
Harper, 26, signed a 13-year, $330 million contract with the Phillies in March. The six-time All-Star and former National League MVP is hitting .333 with two home runs and two RBIs through three games with his new team.