The foreign secretary defeated former Chancellor Rishi Sunak in a ballot of Conservative members – winning by about 21,000 votes.
Ms Truss set out some of her policies during the leadership campaign and has promised to hit the ground running. Here are some of the key pledges and plans she outlined.
COST OF LIVING
Promises to announce a plan to help people and businesses with soaring energy costs within a week of becoming prime minister
Plans an emergency budget to set out measures that would get the economy growing in order to fund public services and the NHS
Says she will address the crisis by putting money back into people’s pockets, such as by immediately reversing the National Insurance rise
Promises not to revisit the idea of windfall taxes on energy firms and rules out energy rationing this winter
Would suspend what is known as the “green levy” – part of your energy bill that pays for social and green projects
Promises to change taxes to make it easier for people to stay at home to care for children or elderly relatives
Says the Bank of England needs to do more to address inflation, arguing “we haven’t been tough enough on the monetary supply” during a leadership debate
TAX AND SPENDING
Says she will reverse the recent rise in National Insurance, which came into effect in April, and hold an emergency budget by the end of September
Pledges not to bring in any new taxes and to scrap a planned rise in corporation tax – set to increase from 19% to 25% in 2023
Would suspend what is known as the “green levy” – part of your energy bill that pays for social and green projects
Says she will pay for the cuts by spreading the UK’s “Covid debt” over a longer period
Promises to change taxes to make it easier for people to stay at home to care for children or elderly relatives
Wants to create new “low-tax and low-regulation zones” across the country to create hubs for innovation and enterprise
Says she won’t cut public spending unless there is a way to do so that won’t lead to future problems
Would bring target of spending 2.5% of GDP on defence forward to 2026 and introduce a new target of 3% by 2030
CLIMATE
Says she will honour the goal of reaching net zero by 2050 and spoke of “accelerating our transition to net zero” at the COP26 climate summit
Would suspend what is known as the “green levy” – part of your energy bill that pays for social and green projects
Would review the ban on fracking
Says the UK needs to build more nuclear power stations and small modular nuclear reactors and would review the ban on fracking
Wants to protect wildlife and biodiversity better and would launch a new UK survey of wildlife to understand which species are endangered
As environment secretary, she cut subsidies for solar farms calling them “a blight on the landscape”.
BREXIT
Argues she can be trusted with Brexit despite voting Remain in the 2016 referendum
Responsible for introducing the Northern Ireland Protocol Bill, which could override parts of the post-Brexit deal between the UK and the EU
Says UK courts should be the “ultimate arbiter” and that trade between Northern Ireland and Great Britain must be “free-flowing”
Promises to scrap or replace by the end of 2023 EU laws considered to be holding back the economy.
HEALTH AND SOCIAL CARE
Has pledged to divert a greater share of healthcare spending towards helping with social care
Says GP services need to be more accessible in order to reduce the pressures on hospital services
Says there should be more mental health support available in schools
Wants to encourage doctors who came out of retirement to help the NHS during the pandemic “to come back into the profession”
EDUCATION
Pledges to give every child “the best opportunity to succeed” wherever they are from and whatever their background
Would expand existing high-performing academy schools, and replace failing establishments with free schools
Promises parents more childcare around the school day and to widen the range of providers who accept government childcare entitlements
Wants to reform university admission procedures so students apply after getting their A-levels (or equivalent) rather than based on predicted grades
Says students receiving top grades should be invited automatically to apply to Oxford and Cambridge
Wants more mental health support available in schools
Says she wants schools to provide single sex toilets
HOUSING AND PLANNING
Says she would end “Stalinist” housing targets – the government currently wants 300,000 homes built in England every year
Plans to create “opportunity zones” with tax cuts and deregulation, making it easier and quicker to build on brownfield sites
Wants to help first-time buyers by incorporating rental payments into mortgage assessments.