The 48-year-old, from Aberdare in Wales, shared a photo of them sat together and wrote: “Thank you for making me smile again.”
Wallace is an actress and producer with credits in science-fiction film Loveland and supernatural thriller Bloodline.
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Gruffudd hit the headlines in January after his wife Alice Evans tweeted to say he was leaving her and their two children.
The post was later deleted.
He filed for divorce at Los Angeles Superior Court in early March.
Gruffudd and Evans, 50, met on the set of 2000 film 102 Dalmatians and had been married since 2007.
Gruffudd had a role in 1997 blockbuster Titanic, before starring in ITV series Hornblower from 1998-2003.
His other film roles include Black Hawk Down, Horrible Bosses and San Andreas.Internet Explorer Channel Network