The nine-time major champion urged golf organizations to start random testing.
'It's absolutely essential that we do that,' Player said at Carnoustie, where he won the British Open in 1968. 'We're dreaming if we think it's not going to come into golf.'
Player says it already has.
'Whether it's HGH, whether it's Creatine or whether it's steroids, I know for a fact that some golfers are doing it,' he said.
Asked how he knew for certain, he said one golfer told him.
'I took an oath prior to him telling me -- I won't tell you where -- but he told me what he did, and I could see this massive change in him,' Player said. 'And somebody else told me something, that I also promised I wouldn't tell, that verified others had done it.'
The cryptic accusation put the Royal & Ancient Golf Club on the defensive over why it does not have drug testing at the British Open, with chief executive Peter Dawson being asked whether he was concerned that its winner could be using steroids.
'I don't know if Gary Player is right about golfers being on drugs, frankly, so I really can't comment,' Dawson said. 'One thing I do know is that we're not drug testing here at the Open championship this week, so just how that would be identified, I'm not sure.'
The R&A and the USGA used drug testing for the first time last November at the World Amateur Team Championship in South Africa, where all 12 samples came back negative.
The LPGA Tour has said it will start drug testing next year and recently disclosed a list of banned substances to its players. The European Tour and the PGA TOUR are working on drug policies, which PGA Tour commissioner Tim Finchem said likely would lead to a testing program, although its first step is developing a list of drugs that would be banned.
Player did not say when this conversation with the purported steroid user took place, nor did he say whether he shared this information with tour officials.
'I can't speak to Gary's remarks,' PGA TOUR spokesman Ty Votaw said. 'I can't disagree and I can't agree with him. They are his remarks that's he is free to make.'
None of the top players concurred with Player.
'He knows two (players)? I know zero,' six-time major winner Nick Faldo said. 'Never heard of anybody.'
Tiger Woods, who last year said he would like to see the PGA TOUR begin drug testing as quickly as possible, was asked Tuesday if he would be surprised if a golfer tested positive for drug use.
'If anything, probably out here it would be testing positive for maybe being hung over a little bit,' Woods said. 'But that's about it. I know some guys have taken Medrol packs for inflammation in their wrists, but other than that, I really don't see anybody doing anything, or have heard anybody doing anything.'
Phil Mickelson also said he thought golfers were clean.
'I don't think there's even a remote chance that will happen,' he said of a golfer testing positive for steroids.
The R&A and USGA are the governing bodies for golf around the world, although both Dawson and USGA executive director David Fay have said that leaders of the professional tours should take the lead in developing a drug policy.
Player said he felt 50 to 60 percent of athletes in the world are using performance-enhancing drugs. When asked for a number of golfers, he estimated 10.
'I might be way out,' Player said. 'Definitely not going to be lower, but might be a hell of a lot more.'