The head of Berkshire Hathaway is among a group that includes Dan Gilbert, the billionaire founder of Quicken Loans.
US news reports and the Financial Times said Mr Buffett co-signed a letter from the group and pledged money for a bid.
Verizon, private equity firms, and the owner of the Daily Mail have expressed an interest in Yahoo's assets.
Last month the search company, hit by falling advertising and weakening investor confidence, shortlisted about 10 potential buyers.
News of Mr Buffett's interest could improve Yahoo's sale hopes. He admitted at Berkshire's annual meeting last month that his investment firm had been slow to get involved the new tech industry.
Mr Buffett, whose firm has a stake in IBM, said in an interview on 2 May that Yahoo's business had deteriorated significantly and that "something has to change there". But he did not mention any potential bid.
Reuters, which first reported Mr Buffett's involvement, pointed out that Susan Decker, who worked at Yahoo in several senior roles between 2000 and 2009, is now a director on Berkshire's board.
The news agency quoted an interview she gave to CNBC television on 29 April: "I hope the next owner can do something to revitalise the spirit of the core things that made Yahoo very, very unique and create a distinction in consumers' minds about why they love Yahoo still.
"It will be helpful if it is private or part of a much larger corporation to achieve that," she said.
However, Verizon is thought to remain the frontrunner in the bidding for Yahoo.
The US telecoms giant would merge Yahoo with AOL, which it bought for $4.4bn (Â£3bn) last year to create a digital group capable of taking on the likes of Google and Facebook.

Cox told US chatshow host Jimmy Kimmel how she had struggled with the way the Londonderry man pronounces certain words.
The pair met in 2013 through mutual friend Ed Sheeran, and announced their engagement last year.
The actress showed a video clip of her family attempting to say the phrase "how now brown cow", with mixed results.
"My fiancé is from Northern Ireland and it's a great place - nice people there," she said.
"But you know, I'm from Alabama and we're real southern.
"He's got this beautiful accent - so we were celebrating our engagement a year ago and in Northern Ireland you say the word 'now', not like n-o-w but it's like n-a-u-w.
"It's weird and it's hard to do - I cannot get it.
"So we were sitting at this table and I was trying to get these southerners to try to say, 'how now brown cow' in the way he says it."
Kimmel joked: "I love that Johnny is just sitting there being abused by your whole family, thinking 'I came here from Northern Ireland to be made fun of?'"
She laughed: "He wanted to go home after that."

Regina Lopez said she was disappointed that "business interests" had "manoeuvred the decision".
She had been in the position for 10 months but had not been confirmed.
Congressional confirmations in the Philippines often happen months after ministers start work.
The decision made by the country's Commission on Appointments is final.
Backed by President Rodrigo Duterte, Ms Lopez had ordered 22 out of 41 mines to be closed, and banned open-pit mines.
She also cancelled contracts for undeveloped mines, and said environmental damage was harming the country's poor.
Following the vote, she said: "I'm disappointed because clearly there are business influences that have manoeuvred the decision. And, from my point of view, government should make decisions primarily on the filter of the common good.
"And when business interests take dominant influence in a way that even adversely affects our people, that's not good at all."
The Chamber of Mines of The Philippines said it would seek to undo her measures when a new minister was in post.
"Those have no legal foundation," chamber spokesman Ronald Recidoro said.
"There were no proper consultations held. And more importantly it's really out to kill the mining industry."
Yeb SaÃ±o of Greenpeace Southeast Asia said the sacking "shows how destructive industries continue to hold Philippine lawmakers by their necks."
A spokesman for the president said no replacement had yet been lined up.
Ms Lopez's supporters, who call her an earth warrior, have organised candlelit vigils to mark the end of her tenure.
Legendary investor Warren Buffett is backing a consortium bidding for Yahoo's core internet assets, according to reports.
[NEXT_CONCEPT]
Former Friends star Courteney Cox has been trying to teach her family how to speak in an authentic Northern Ireland accent like her fiancé, Snow Patrol musician Johnny McDaid.
[NEXT_CONCEPT]
Lawmakers in the Philippines have removed the environmental secretary, who angered the mining industry by ordering the closure of more than half the country's existing mines.