Nearly half of 37.2 million adults living with HIV are women, figures show.
Women are more vulnerable to infection
The steepest increases have been in East Asia, Eastern Europe and Central Asia, with rates in women outstripping those in men in some regions.
As well as being biologically more vulnerable to infection than men, women are forced to have sex through violence or financial reasons, said UNAIDS.
The number of people living with HIV globally has also reached its highest.
There is an estimated 39.4 million people living with HIV globally, up from an estimated 36.6 million in 2002, fuelled mainly by unprotected sex and intravenous drug use.
Over the past two years the number of women living with HIV has risen in every region of the world.
Women now make up nearly half of the 37.2 million adults aged 15-49 living with HIV worldwide.
In sub-Saharan Africa about 60% of those with HIV are women.
When you look at only young people aged 15-24, this rises to 75%.
Over the past two years alone, the number of women infected in East Asia has increased by 56%.
In Eastern Europe and Central Asia the number has increased by 48%.
UNAIDS said there were a number of reasons for the gender trend.
Women are more physically susceptible to HIV infection than men, and male-to-female transmission during sex is about twice as likely to occur as female-to-male transmission.
Millions of women and girls are becoming sexually active each day with no access to preventive services, says UNAIDS.
Many in monogamous relationships do not believe they have the right to ask their husbands to use a condom, even if he had proven himself to be unfaithful and was HIV-positive.
In Asia , the Caribbean and sub-Saharan Africa, using sex as a commodity in exchange for goods, services or other basic necessities is becoming common among women.
Others are violently forced to have unprotected sex.
UNAIDS hopes to reverse the trend through its Global Coalition on Women and Aids.
Dr Peter Piot, UNAIDS executive director, said: "Strategies to address gender inequalities are urgently needed if we want a realistic chance at turning back the epidemic.
"Concrete action is needed to prevent violence against women and ensure access to property and inheritance rights, basic education and employment rights for women and girls."
He said the Aids epidemic was entering a new phase thanks to investment.
"There is still continuing expansion of the epidemic, but also we are seeing more and more countries and even big cities, such as in Eastern Africa or in Cambodia or in Brazil, where the number of new infections is actually declining, particularly in young people.
"Secondly, there is far more money now.
"It is now a matter of seeing that money work and I think we will soon see the impact."
Dr Lee Jong-Wook, director general of WHO, said: "Only by linking prevention and treatment can the global spread of Aids be halted."
Nick Partridge, chief executive of the Terrence Higgins Trust, said: " Once again, women are shown to be bearing the brunt of the epidemic globally, both in terms of living with HIV and as carers.
"Continued research into prevention methods such as microbicides which give women control over their sex lives, alongside education and treatment, is vital in order to help slow the spread of the virus."
The Department for International Development said it was working to reverse the spread of Aids in the developing world, especially amongst young women.
It said it would be spending over £1.5 billion on HIV prevention, treatment and care and medical research over the next three years.