The Norwegian Nobel Committee honoured the three women "for their non-violent struggle for the safety of women and for women's rights to full participation in peace-building work."
Johnson Sirleaf, 72, is a Harvard-trained economist who became Africa's first democratically elected female President in 2005. She faces a Presidential poll this month. She was seen as a reformer and peacemaker in Liberia when she took office. But recently, opponents in the presidential campaign have accused her of buying votes and using government funds to campaign.
Her camp denies the charges. Liberia was ravaged by civil wars for years until 2003. The country is still struggling to maintain a fragile peace with the help of UN peacekeepers. Liberian peace activist Leymah Gbowee, organised a group of Christian and Muslim women to challenge Liberia's warlords.
In 2009 she won a Profile in Courage Award, an honour named for a 1957 Pulitzer Prize-winning book written by John F Kennedy, for her work in emboldening women in Liberia.
Tawakul Karman, 32, is a mother of three. She headed the Women Journalists without Chains, a human rights group for journalists. She had been a leading figure in organising the protests in Yemen that opposed the rule of President Ali Abdullah Saleh and which kicked off in late January.
Karman's father is a former legal affairs minister under Saleh. She is a journalist and member of Islah party, an Islamic party. "We cannot achieve democracy and lasting peace in the world unless women obtain the same opportunities as men to influence developments at all levels of society," the prize committee said.