Knight had previously represented Berkshire, having come through the Devon pathway, and captained Western Storm to two Kia Super League titles in the regional system. She was appointed to the Somerset board in an advisory role earlier this year.
“Somerset is a club that holds a special place in my heart, so I’m incredibly excited about this opportunity,” Knight said. “The South West has played an integral role in my journey and it’s a privilege to be able to represent the region that means so much to me.
“We have a genuine opportunity to create something special. This is the first Somerset women’s team of the professional era, and we have a duty to those who have gone before and laid the foundations to make sure that we are a team that competes on the field and inspires young people across the region. There is a special heritage surrounding the club and that is something that we will be taking very seriously.
“The Cooper Associates County Ground is a special place to play, and I can’t wait for some of the younger players to experience the atmosphere that the members and supporters can generate. The Taunton crowd is passionate and knowledgeable, and we will be doing everything that we can to win games of cricket and give them something to celebrate.”
A World Cup-winner, Knight has captained England across formats since 2016, and is currently leading the team on their tour of South Africa.
Somerset Women head coach, Trevor Griffin, said: “Heather is a player I’ve worked with for many years, and I’m delighted she will be playing for Somerset when her England commitments allow. She is a true professional who impacts games both on and off the pitch and is an invaluable addition to any side.
“I’m sure our supporters will be looking forward to seeing Heather in a Somerset shirt and winning games of cricket for us.”
Somerset’s director of cricket, Andy Hurry, added: “Heather is a world-class performer who has shown her quality on the biggest stages across the world. She is hugely respected within and outside the game, and the experience she can share both on and off the field will have a major impact on the team, the club and the region.
“It’s an honour to have the England captain committed to the club, and there is no doubt that she will inspire future generations of Somerset players, members and supporters.”