Pierce, 62, took to photo-sharing application Instagram to share a picture of Sean and the newborn Marley May Cassandra, reported Contactmusic.
"My darling Sean With his baby girl Marley May Cassandra what utter joy to be her grandpa... Beautiful life (sic)," he captioned the picture.
Sean is Pierce's son with late wife Cassandra, who died from ovarian cancer in 1991.
Brosnan, who married twice and has five children, has two other grandchildren, Isabella and Lucas.