Speaking alongside Bill Gates to Columbia Business School students, Mr Buffett said: "The financial panic is behind us.
"The bottom has come in stocks. Don't pass on something that's attractive today."
Mr Buffett went on to praise the actions of the Bush administration in the wake of the crash of Lehman Brothers last year, admitting that only the government had the economic might to have saved things.
But he added that in future, there should be "more downside" for any organization that needs a taxpayer bailout.
Earlier this month, Mr Buffett's investment firm Berkshire Hathaway announced it was to spend $44 billion on a takeover of Burlington Northern Santa Fe, the railroad haulage company.
At the time of the deal, which is the biggest in Berkshire Hathaway's history, Mr Buffett described it as an "all-in wager" on the economic future of the US.
By Claire Archer