Its rise to the summit comes just three months after Google's OS trailed iOS by more than 1.5 million subscribers.
The firm's numbers taken from its MobiLens report for March have Android on 4.6 million users to the 3.9 million on iOS, with Google's userbase up from the 2.2 million it boasted at the end of 2010.
Hopping on with Android
As a whole, comScore notes that smartphone subscriptions are up 71 percent over the course of the six month period between September and March, with adoption continuing to accelerate across the country.
"The surge in smartphone adoption during the past six months has been led by Android devices, which has increased its installed base by nearly 3.8 million users while the market as a whole has grown by 4.1 million users," the firm notes, adding Android's userbase has more than double in the past three months.
Both Android and iOS account for 87 percent of the smartphone market in Japan, with other platforms barely registering any share.
However, while Google can claim to be the market leader in terms of OS, Apple's 32GB iPhone 4 remains the best selling smartphone device in the region, ahead of Sony Ericsson's Xperia (the specific handset not named) and the 16GB iPhone 4.
[source: comScore]
