Saudi Arabia, Chad and Nigeria were elected by the UN General Assembly to serve a two-year term on the UN Security Council as human rights groups called for all three countries to improve their records.
Chile and Lithuania also won seats on the 15-member council on Thursday.
There are five veto-wielding permanent council members - the United States, Britain, France, Russia and China - and 10 temporary members without veto power.
The group elected on Thursday will replace Azerbaijan, Guatemala, Pakistan, Morocco and Togo on the Security Council on Jan 1, 2014.
They were unopposed, but had to obtain approval from two-thirds of the 193-member General Assembly.
Of the 191 UN members who voted, Lithuania won 187 votes, Chile and Nigeria each picked up 186 votes, Chad secured 184 votes and Saudi Arabia 176 votes.
"Security Council members are routinely called upon to address critical human rights and humanitarian issues," said Hillel Neuer, executive director of UN Watch, a Geneva-based advocacy group that monitors the UN.
"Saudi Arabia and Chad have abysmal records on human rights."
Saudi Arabia, a patriarchal society that applies an austere version of Sunni Islam, has repeatedly been slammed for its record on women's rights. Last month it topped a World Bank list of countries with laws that limit women's economic potential.
Saudi Arabia, which is also campaigning to be elected to the UN Human Rights Council, grants fathers guardianship over their daughters, giving them control over who they can marry and when, and it is also the only country in the world where women are barred from driving.
Human Rights Watch said taking up these prominent UN positions should spur Riyadh to "clean up its act".
A senior Security Council diplomat, speaking on condition of anonymity, welcomed the election of a key Middle East country as the world attempts to bring to an end a 2-1/2-year-old civil war in Syria that has killed more than 100,000.