Police were called to Love Ranch brothel in Crystal, Nevada, after he was found unresponsive on Tuesday.
The American had to be driven to hospital because he was too tall to be airlifted in a helicopter.
Mr Odom, 35, has played basketball for Miami, Dallas and the Los Angeles Lakers and Clippers.
He also played for the US in the 2004 Athens Olympics, winning the bronze medal.
His condition is unknown but well-wishers tweeted their support following the news.
Mr Odom, who was married to reality television star Khloe Kardashian, has long struggled with alcohol and drug addiction.
He was involved in a drink-driving case in 2013 and was suspended from the NBA for violating its anti-drug policy in 2000.
He and Ms Kardashian were married in 2009 in a ceremony that was taped for US television network E!.
They divorced in 2013.
Odom won the NBA championship with the Lakers in 2009 and 2010 and was named NBA Sixth Man of the Year in 2011. He last played in the NBA in 2013 with the Clippers.

Giovanni Dixon, 17, of Norwood, stabbed James Hunter in the heart after the 18-year-old tried to protect his friend.
The Old Bailey heard Dixon and his friends had set upon Ezekiel Andrews with knives last May and Mr Hunter stepped in to protect him.
Dixon was found guilty of murder and wounding and was sentenced to a minimum of 16 years.
The court heard Mr Andrews, 18, who was described as a "tall and somewhat awkward boy", was punched to the ground, kicked and stabbed three times in the buttocks, following a perceived slight to the gang's friend Jacob Ayoade in the weeks before.
Mr Hunter, 18, suggested they fight "one on one" but Dixon rejected that idea and instead stabbed him in the heart on Wells Park Road.
The court heard Dixon "jovially" described what he had done as he made his getaway on a bus with his friends Jerome Catterall, 18, Jason Odumusi, 17, and Benedict Leka, 16.
Judge Rebecca Poulet QC said the boys' behaviour demonstrated a "preoccupation almost bordering on obsession with knives".
Catterall, of Sydenham and Odumusi, from Catford, were cleared of murder but convicted of wounding and sentenced to four and a half years each while Leka, from Forest Hill, who was 15 at the time, received four years for the same offence.
Ayoade, from Forest Hill, was sentenced to three years for encouraging the offence and possessing a knife.
Judge Poulet lifted a ban on naming all the defendants in light of their convictions.

The 23-year-old younger brother of Sunderland's Wes Brown, had been on trial with the Shakers.
Brown has also had spells at Bradford, Doncaster, Oldham, Coventry, Ipswich, Watford and Carlisle, making over 50 appearances in the Football League.
He is available for Bury's League One season opener as they travel to Doncaster on Saturday.
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Ex-NBA basketball player Lamar Odom is being treated in hospital after being found unconscious at a brothel in Nevada, authorities have said.
[NEXT_CONCEPT]
A teenager "obsessed" with knives who bragged about stabbing a boy in south-east London has been jailed for life.
[NEXT_CONCEPT]
League One side Bury have signed former Barnsley defender Reece Brown on a six-month contract.