Apple became the first US company to top the $800 billion mark in market capitalisation last night, slightly more than two years after it crossed the $700 billion threshold.
The iPhone maker's shares have gained 33% this year and almost 50% since the US election in November.
The company represents about 4% of the $21.7 trillion that makes up the entire S&P 500 index.
Apple accounted for as much as 4.9% of the S&P 500 in September 2012 but is a smaller percentage now as the index as a whole has rallied more than 7% this year.
If Apple continues on its growth path, the company will top the $1 trillion market cap level later this year.
Stock buybacks have also bolstered Apple shares, with the company reducing its actual share count by 20.9% and the average diluted shares outstanding by 20.5% over the past four years, according to Standard & Poor's data.
The median price target on Apple is $160, up from the $140 median three months ago. Shares closed at $153.99 last night.
Billionaire Warren Buffett's Berkshire Hathaway has disclosed a stake of roughly $20 billion in Apple.
The businessman said earlier this week that he had grown more fond of the company because he could "very easily determine" the iPhone maker's competitive position "and who is trying to chase them."
Meanwhile, Apple has acquired a sleep tracking app and hardware maker Beddit, the Finnish startup said on its website.
The terms of the deal were not disclosed.
"Beddit has been acquired by Apple. Your personal data will be collected, used and disclosed in accordance with the Apple Privacy Policy," the company said.
Beddit manufactures a monitoring device which tracks heart rate, breathing and sleep time when placed on a bed.
The monitor, which sells for $149.95 in Apple's online store, transfers the data to Beddit's iPhone app for analysis.
