A statement from the presidency confirmed that the bill, which it said "consolidates various laws relating to marriage," had been signed into law.
The bill, which amended previous marriage legislation, was passed by parliament last month following heated late-night debates that saw female lawmakers storm out of the room in protest.
Supporters of the new law say it brings civil legislation in line with customary practices. While polygamous unions are common among many Kenyan tribal groups, multiple partner unions were previously not recognised by the state.
Speaking to the Associated Press on Tuesday, lawyer Judy Thongori said previous marriage laws were made by British colonialists before Kenya gained independence in 1963 and did not reflect the value systems of indigenous populations.
Kenyatta – who faces crimes against humanity charges at the ICC (International Criminal Court) for ethnic violence following the 2007 presidential election – was elected into office last year after running an anti-Western campaign that particularly targeted the US and Kenya’s former colonial power.
The presidential statement released Tuesday noted that, “Marriage is the voluntary union of a man and a woman, whether in a monogamous or polygamous union."
Kenyan women however are not permitted to marry more than one man.
What do the scriptures say?
The new legislation has been mired in controversy and has been condemned by Kenyan women’s rights as well as church groups.
The National Council of Churches in Kenya (NCCK), which groups more than 40 churches and Christian organisations from across the east African nation, has also spoken out against the bill.
"The tone of that bill, if it becomes law, would be demeaning to women since it does not respect the principle of equality of spouses in the institution of marriage," Archbishop Timothy Ndambuki from the NCCK was quoted by Kenya's Standard newspaper as saying before Kenyatta signed the bill into law.
But the acrimonious debate in Kenya’s male-dominated parliament saw the occasional quoting of scriptures to support polygamy.
“I want my Christian brothers to read the Old Testament; King David and King Solomon never consulted anybody to marry a second wife,” the Standard quoted parliamentarian Aden Duale as saying.
But his female colleagues vehemently disagreed, with MP Soipan Tuya telling parliament last month that, “we know that men are afraid of women's tongues more than anything else," before adding, "But at the end of the day, if you are the man of the house, and you choose to bring on another party – and they may be two or three – I think it behoves you to be man enough to agree that your wife and family should know."
‘When you marry an African woman…’
Traditionally, Kenyan men were expected to receive the consent of their first wives before taking on additional wives. An initial version of the bill had initially given a wife the right to veto the husband's choice, but male members of parliament overcame party divisions to push through a text that dropped this clause.
“When you marry an African woman, she must know the second one is on the way, and a third wife... this is Africa," MP Junet Mohammed told the house last month.
The new law does not put a cap on how many wives a man can have.
Kenyan women’s rights advocates have vowed to mount a campaign against the new move and the national Federation of Women Lawyers (FIDA Kenya) has said it would mount a legal challenge against the law.
Supporters of the new law say it abolishes the practice of unofficial traditional marriages which were not recognised by the state and failed to provide women support if the relationship ended.
Parliamentarians rejected a measure that entitled wives to 30% of matrimonial property upon divorce. The new law grants women equal property and inheritance rights.
But critics have noted that polygamous marriages complicate the division of matrimonial properties and could put women at a severe disadvantage.
Kenyan President Uhuru Kenyatta - © AFP