Northern Ireland Secretary Karen Bradley said she has left the Government.
Ms Bradley went into the role last year after James Brokenshire left the role due to ill health.
In a statement, Ms Bradley said: “It has been an enormous privilege to serve as Secretary of State for Northern Ireland and represent this special nation and integral part of our precious union.
“I would like to pay tribute to Theresa May for her continued support and commitment to ensuring the unique needs of Northern Ireland were met and respected in full.
She said she was proud of the achievements over the past 18 months, and said: “I personally regret that I will not conclude the current talks process but I am honoured to have led the process over the past 12 weeks.
“The political parties in Northern Ireland have made important progress and shown a new determination to overcome their differences and reach an agreement.
“I now pass this tremendous responsibility over to my successor with every best wish and my full support to conclude the process.”
A number of Theresa May's cabinet members resigned in the days leading up to Boris Johnson's victory in the Conservative Leadership Contest and others have followed suit since Mr Johnson met the Queen earlier this afternoon to formally take on his new post.
Philip Hammond, Rory Stewart and David Gauke all resigned from the front bench today and Liam Fox and Greg Clark both said Twitter that they would be leaving the government.
- Additional reporting Press Association