The 70-year-old died along with his wife Elaine, 69, after being killed while on holiday in Sousse.
The pair were two of the 38 people killed when 23-year-old Seifeddine Rezgui targeted tourists with gunfire on the Tunisian beach.
Denis and his wife, who lived in Blackpool, had been staying at the Imperial Marhaba Hotel after travelling there on Wednesday.
He was a winger who broke into the Birmingham first team at the age of 16 in 1961.
His last appearance for the Blues first team came during the 1971/72 season, and he retired from professional football in 1972 at the age of 27.
He played in 87 games for Birmingham, scoring 18 goals.
Son-in-law Daniel Clifford, of Blackpool, said: “They only arrived on Wednesday and texted us on Thursday to say they were all fine and starting to enjoy their holiday.”
Blackpool North MP Paul Maynard confirmed to the House of Commons on Monday that two of his constituents had died in the attack.
And Gordon Marsden, MP for Blackpool South, tweeted: “Very sad to hear Denis and Elaine Thwaites among those killed in the terrorist attack in Sousse – condolences to their family.”