The 65-year-old actress shared her joy with her 1.28 million followers, posting a picture of son William True, 23, with the new baby resting on his chest and writing alongside, 'When your son has a son...bliss...yes this is my secret happy news as promised ...Welcome Waylon Tripp Parker.'
Her fans were quick to respond with congratulations. One exclaimed 'How awesome!!! Congrats to you all!! Love that name!', whilst another agreed 'Being a grandmother is the most amazing experience, congratulations Kirstie, enjoy every minute of it.'
When your son has a son...bliss...yes this is my secret happy news as promised ...Welcome Waylon Tripp Parker pic.twitter.com/hTvfyppX4O— Kirstie Alley (@kirstiealley) June 21, 2016
Shortly afterwards, Kirstie couldn't resist posting another snap of the adorable new arrival, adding 'Just GOTTA send one more..proud Gammy'.
Just GOTTA send one more..proud Gammy pic.twitter.com/jK502lNe82— Kirstie Alley (@kirstiealley) June 21, 2016
Waylon is the first grandchild for the TV star, who is mum to William and daughter Lillie Price, 22. The pair were adopted by Kirstie and her ex-husband Parker Stevenson, whom she married in 1983. William joined the family in 1992, and was closely followed by Lillie in 1994.
Kirstie and Parker divorced in 1997, but continued to share joint custody of their two children.
The family have had a lot to overcome in recent years, after Lillie's fiance Nick Trela was tragically killed in a motorcycle accident in LA in 2014. The couple had been dating for four years at the time, and had been planning their wedding.
The incident was responsible for both children's conversion to Scientology, the religion which Kirstie also follows.
'It was their choice,' the actress said in 2015. 'It really helped them a lot so that's good.'
Calling the loss of Nick 'the worst thing that ever happened in our lives', she added, 'He was clearly one of the most awesome men I have ever met in my life. He could do anything… He would build me trails to get to the Lemur compound. The loss is so great, because if I lean on the wall, he built the wall.'