On the company's official Twitter feed, it confirmed there was a problem, calling it "a feed delivery issue," and said it was working to fix it.
The downed service quickly prompted complaints and jokes on Twitter and other sites, often linked to the popularity of sharing photos of meals with the service.
"RED ALERT! We are at DEFCON 1," Technology news site TechCrunch posted in a humorous take, alongside an image of a man on a cell phone and the caption "Instagram is down. Just describe your lunch to me."
Twitter users seemed split between frustration and amusement at others' frustration.
"Instagram is down but my self esteem isn't b/c now I don't have to see selfies from all the HoTtTt ppl I follow," wrote one user from the US state of Texas.
Another from Britain wrote "The fact that Instagram is down actually hurts me inside. This is a problem."
And a third in the US state of Indiana, wrote: "Instagram is down? Whatever will we do? OH I KNOW YOU COULD READ A DAMN BOOK OR ACTUALLY GET SOMETHING ACCOMPLISHED."
The company did not immediately clarify what had caused the problem or say when it would be fixed.