Cobb was selected by Welsh Fire in Tuesday morning's draft for the new competition, which will be launched this summer.
The County's white ball skipper joins red ball captain Adam Rossington in the squads for the eight-team tournament, with Rossington having already been selected by London Spirit.
Cobb joins former Northants team-mate Ben Duckett in the in Welsh Fire squad, who will be based at The Swalec Stadium in Cardiff.
The 30-year-old missed out on the original draft selections a year ago, with the launch of The Hundred then being postponed due to the Covid-19 pandemic.
But he has been selected this time round, and will team up with the likes of England World Cup winners Jonny Bairstow and Liam Plunkett, as well as Ollie Pope, Jake Ball and Australian international Jhye Richardson.
Cobb will also get the chance to play alongside West Indian Kieron Pollard, who was denied the chance to play for the Steelbacks in the Blast last summer due to the tournament being delayed by coronavirus.
The Fire will be coached by Gary Kirsten.
Big-hitting batter and right-arm spinner Cobb is a two-time winner of the T20 Blast in England, having claimed the man-of-the-match award in finals wins for Leicestershire Foxes in 2011, and then the Steelbacks in 2016.
The news means that Cobb, along with Rossington, will now not be available for the Steelbacks' Royal London One Day Cup group campaign, as that competition runs at the same time as The Hundred.
Northants will also be without head coach David Ripley for the Royal London One Day Cup, as he is part of the London Spirit coaching set-up, alongside Aussie legend Shane Warne.
There are still further draft selections to be announced through the morning, while each of the eight teams will also get to select a 'Vitality Blast Wildcard Pick' once the group stages of that competition have been completed.
Crowds are set to be allowed in to watch The Hindred once it starts in July, following the Government announcement of the road map out of lockdown on Monday.
Welsh Fire men's squad: Ollie Pope, Jonny Bairstow, Kieron Pollard, Jhye Richardson, Tom Banton, Ben Duckett, Jake Ball, Qais Ahmad, Liam Plunkett, David Payne, Ryan Higgins, Ian Cockbain, Josh Cobb, Matthew Critchley, David Lloyd (plus Wildcard pick)