May, 59, is Britain's second women prime minister after Margaret Thatcher who was in power from 1979 to 1990.
Below is a list of countries which have women as heads of government or elected heads of state with United Nations data showing about half of these are the first women to hold their country's highest office.
The date in brackets is the year they took office. The list excludes figurehead monarchs.
* BANGLADESH: Prime Minister Sheikh Hasina (2009)
* CHILE: President Michelle Bachelet (2014)
* CROATIA: President Kolinda Grabar-Kitarovic (2015)
* GERMANY: Chancellor Angela Merkel (2005)
* LIBERIA: President Ellen Johnson Sirleaf (2006)
* LITHUANIA: President Dalia Grybauskaite (2009)
* MALTA: President Marie-Louise Coleiro Preca (2014)
* MARSHALL ISLANDS: President Hilda Heine (2016)
* MAURITIUS: President Ameenah Gurib-Fakim (2015)
* NAMIBIA: Prime Minister Saara Kuugongelwa-Amadhila (2015)
* NEPAL: President Bidhya Devi Bhandari (2015)
* NORWAY: Prime Minister Erna Solberg (2013)
* POLAND: Prime Minister Beata Szydlo (2015)
* SOUTH KOREA: President Park Geun-hye (2013)
* TAIWAN: President Tsai Ing-wen (2016)