The Blue Peter and Sky star said on Instagram that their daughter was born early after Derrina "fell rapidly ill" and weighed just over three pounds.
He also shared snaps of Derrina and their new arrival, as well as some of his 13-year-old son Ethan, who is from his marriage to late wife Gemma.
"Not everything in life goes quite according to plan," Thomas wrote.
"One moment we think we’ve got eight weeks until our little one arrives; the next, after Derrina fell rapidly ill, we’re suddenly welcoming into the world a very tiny, but hugely precious baby girl weighing just over three pounds.
"The #neonatal team at #stokemandeville hospital were simply incredible and held our hands physically and in spirit the whole way through a very fear filled and stressful day yesterday."
The 49-year-old went on: "I cannot express how proud and blessed I feel right now - I have a beautiful, brave wife in @derrina, a precious son and now a tiny, wonderful daughter and sister for our boy Ethan.
"We have a long road ahead of being in hospital for the next few weeks; but we have the best team around us."
"Thank you Stoke Mandeville, thank you #nhs for being there for us and thank you God for blessing us as a family so much," he added.
Thomas became a widower and single parent to Ethan in 2017 when his first wife Gemma died just three days after being diagnosed with acute myeloid leukaemia.
He previously told how having to tell his son that his mum had died was something he wouldn't wish on his worst enemy.
“You can’t dress it up. I just said, ‘Mummy’s died’, and he collapsed on to the floor," he said during an appearance on This Morning in 2018.
"I collapsed with him. I just held him."
Thomas went on to date Derrina and in July 2021 he shared that they had tied the knot, with Ethan acting as one of the best men.
Watch: Simon Thomas weds girlfriend Derrina Jebb