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SAO PAULO, June 25 (Reuters) - Brazil's stocks jumped on Thursday tracking sharp gains in U.S. stocks, while a rally in commodity prices boosted heavyweights Petrobras and Vale.

Brazil's Bovespa stock index .BVSP jumped 3.71 percent to 51,514.81 points -- the biggest one day gain in more than a month. The index rallied in March, April and May but is down so far in June.

The real currency (BRBY: Quote, Profile, Research) strengthened 1.8 percent to 1.946 per dollar as the optimism raised appetite for riskier emerging market currencies.

Stock gains mirrored those on Wall Street where a surprising profit increase from Bed Bath & Beyond boosted retailers and investors were relieved after Federal Reserve Chairman Ben Bernanke's congressional questioning ended.

Bernanke faced his toughest grilling yet by U.S. lawmakers on Thursday, where he said he had never threatened to fire Bank of America's management if they pulled the plug on a planned merger with Merrill Lynch. See [ID:nN25267831].

At the stock exchange, oil firm Petrobras (PETR4.SA: Quote, Profile, Research) was the biggest weighted gainer, up 4.03 percent at 32.25 reais as oil prices surged.

Mining giant Vale (VALE5.SA: Quote, Profile, Research) also jumped 4.1 percent to 30.69 reais as copper prices gained 1.7 percent. The company's chief executive Roger Agnelli said on Thursday Vale was in no rush to finish iron ore price talks with Chinese steelmakers and saw little possibility they could buy ore on spot markets. See [ID:nN25263152].

Higher metal prices also boosted steelmakers with Usiminas (USIM5.SA: Quote, Profile, Research), Gerdau (GGBR4.SA: Quote, Profile, Research) and CSN (CSNA3.SA: Quote, Profile, Research) gaining between 4.2 and 5.7 percent.