"There's finally a realization on the part of investors that this is not a one-trick pony," Candor Fitzgerald market analyst Youssef Squali said.
The Financial Times reported Google's value rose to $249.1 billion Monday, nearly $2 billion more than Microsoft.
That makes Apple, Google and then Microsoft one, two and three in value among technical firms.
In late August, Apple became the most valuable company in history with share prices pushing the firm to a $623.1 billion valuation.
Microsoft, which has slid back since, had held the record with a value of $620.6 billion, which was the company's worth in 1999.
The second most valuable company, which Apple surpassed in August 2011, is ExxonMobil, valued at $406 billion.
But among technical firms, Google has survived doubts about its diversity with its Android operating system for mobile phones, which has helped it jump to the head of the pack in mobile advertising. It now commands about half of that market, the Times said.
Citigroup analyst Mark Mahaney, who evaluates Internet firms, said Google is on track to surpass Facebook in online display advertising, which could bring the company $6 billion in revenue in 2012.