The UN health agency said the B.1.617 variant of Covid-19 first found in India last October seemed to be transmitting more easily.
"There is some available information to suggest increased transmissibility of the B.1.617," Maria Van Kerkhove, the WHO's lead on Covid-19, told reporters.
"As such, we are classifying this as a variant of concern at the global level," she said.
She also pointed to early studies "suggesting that there is some reduced neutralisation", meaning that antibodies appeared to have less impact on the variant in small-sample lab studies.
The WHO insisted though that it was far too early to interpret this to mean that the variant might have more resistance to vaccine protections.
"Based on current data, the COVID-19 vaccines remain effective at preventing disease and death in people infected with this variant," it said in a statement.
More details would be provided about the variant in the WHO's weekly epidemiological update on Tuesday, Van Kerkhove said.
India, suffering from one of the worst outbreaks in the world, reported nearly 370,000 fresh infections and more than 3,700 new deaths on Monday.
The devastating wave has overwhelmed India's healthcare system, and experts have said the official figures for cases and fatalities are much lower than the actual numbers.
It has for some time been feared that B.1.617 -- which counts several sub-lineages with slightly different mutations and characteristics -- might be contributing to the alarming spread.
But until now, WHO has listed it merely as a "variant of interest".