Speaking to media on behalf of the family, Jermaine said his brother suffered a cardiac arrest, and asked for "privacy at this tough time".
"My brother Michael Joseph Jackson passed away on Thursday June 25 at 2.26pm," Jermaine Jackson said.
"His personal physician, who was with him at the time, attempted to resuscitate my brother, as did the paramedics as they transported him to Ronald Reagan Medical Center in Los Angeles.
"May our love be with you Michael, always. We love you."
Friends of Jackson have led tributes to the "King of Pop" as hundreds of fans gather outside the Medical Center.
Reports of the singer's death began to surface soon after he was taken to the hospital around 7.30am NZST.
A short time ago Ed Winter, the assistant chief coroner for Los Angeles County, confirmed his office had been notified of the death and would handle the investigation.
OK! magazine is reporting another person from the coroner's office, Fred Corral, as saying an autopsy will likely be carried out on Friday local time, and that the cause of death may be known as soon as Friday night local time.
"For Michael to be taken away from us so suddenly at such a young age, I just don't have the words," said Quincy Jones, who produced Thriller.
"He was the consummate entertainer and his contributions and legacy will be felt upon the world forever. I've lost my little brother today, and part of my soul has gone with him."
Singer Dionne Warwick said: "Michael was a friend and undoubtedly one of the world's greatest entertainers that I fortunately had the pleasure of working with. ... We have lost an icon in our industry."
A large crowd has gathered outside the hospital and traffic jams are blocking the area while news helicopters whirr overhead.
The emergency entrance at the hospital, which is near Jackson's rented home, was roped off with police tape.
Elsewhere, fans have gathered on the Hollywood Walk of Fame and in New York's Times Square to pay tribute to Jackson.
"Ladies and gentlemen, Michael Jackson has just died," a woman boarding a Manhattan bus called out, shortly after the news was announced. Immediately many riders reached for their cell phones.
So many people wanted to verify the early reports of Jackson's death that the computers running Google's news section interpreted the fusillade of "Michael Jackson" requests as an automated attack for about half an hour.
In Times Square, a low groan went up in the crowd when a screen flashed that Jackson had died, and people began relaying the news to friends by cell phone.