The four-time Ballon d'Or, 27, took to Instagram to post a picture of his son, Thiago, kissing Antonella Roccuzzo's stomach with the message: "Can't wait to welcome you into our world!!! We love you! Thiagui, mum and dad."
Local press in Argentina had been speculating about Messi's future fatherhood since February with newspaper Clarin saying the couple already knew they were due to have a baby boy who would be named Benjamin.
Messi and Roccuzzo's first son was born in November 2012.