Alphabet has a market cap of $547.1 billion, higher than Apple's $529.3 billion as of 9:45 a.m. ET.
Alphabet was already set to take the title Monday night, before sealing the deal Tuesday morning. At Monday's after-hours levels (which technically reflect an indication, but not the real-world value), Alphabet's market cap hit roughly $570 billion, eclipsing Apple's market cap of about $535 billion. Shares of Apple opened Tuesday down nearly 1 percent.
Chart source: FactSet
The last time Google was more valuable than Apple was in February 2010, when both companies were worth less than $200 billion. At the time, Apple had yet to release its first iPad, the newest iPhone on the market was the 3GS, and the Mac was the company's biggest product line, accounting for one-third of revenue. Steve Jobs was still at the helm.
Apple and Google actually flip-flopped multiple times between 2008 and early 2010, before Apple went on a historic tear, jumping from $180 billion in value to over $650 billion in September 2012. At that point, the two companies were separated by over $400 billion. In 2011, Apple passed Exxon to become the world's most valuable company.
It's hard to believe that Google was the more valuable company from the time of its IPO in 2004 until April 2008. Then iPhone madness began.
Google's latest rise versus Apple began in July. From that point through the end of 2015, its shares soared 44 percent, while Apple's sank 16 percent.
Apple's main problem is its reliance on the iPhone, which now accounts for two-thirds of revenue. It's a massive business, but sales in the fiscal first quarter increased only 1 percent from a year earlier, while iPad and Mac revenue dropped. Investors are concerned that unless Apple changes course and decides to compete with lower cost Android manufacturers on price, the iPhone's best days are in the past.
Meanwhile, Google is convincing investors that in the transition from Web to mobile it will maintain its dominance. According to eMarketer, Google is poised to capture 32 percent of the mobile ad market this year and next, staying well ahead of Facebook, which is around 20 percent. The company generates so much profit from its digital ad business that it can invest in all sorts of potential growth areas, namely autonomous driving and extending life.
—CNBC's Everett Rosenfeld and Chris Hayes contributed to this report.
CORRECTION: An earlier version of this article included a graph with an incorrect scale.