The 31-year-old actor – who rose to fame as a childstar playing Harry Potter’s best friend Ron Weasley – welcomed a daughter with his partner Georgia Groome on 7 May.
Radcliffe, 30, told Watch What Happens Live With Andy Cohen: “‘I texted him the other day and said I was so happy for him and it’s very, very cool.
“It’s also like super weird, for me, to think that we are all at the age where we are all having children, but we definitely are.”
Grint and Radcliffe grew up on screen in the Harry Potter movies, along with Emma Watson, who played their friend Hermione Grainger.
Radcliffe said: “I remember when I turned 30 I remember a lot of people in my life were really depressed to find out that they’re lapped.”
He previously told the Inquirer of Grint’s impending arrival: “I’m incredibly happy for him. He and Georgia have been together for a while. It’s really exciting. It is crazy to think because it’s in my head. It still feels like we’ve basically just met and we are about 16 years old.”
Grint and Angus, Thongs And Perfect Snogging star Groome, 28, have been together since 2011. They announced the news that she was pregnant in April.
A statement from the couple’s publicist said: “Rupert Grint and Georgia Groome are delighted to confirm the birth of their baby girl. We would please ask that you respect their privacy at this very special time.”
Radcliffe is in a relationship with actress Erin Darke. They met when they starred together in Kill Your Darlings in 2013.
The actor was starring in a production of Endgame with Alan Cumming at London's Old Vic Theatre, which was cancelled early due to the coronavirus outbreak.
Radcliffe admitted he is worried about maintaining social distancing rules when lockdown ends.
He admitted: “I really am going to find it very hard not to shake people’s hands. Because I am a compulsive handshaker and I feel very rude not doing it. But I guess we’re not going to be doing that for a while, so I’m just going to feel like I’m being HORRIBLE!”