SAN FRANCISCO (Reuters) - Retailer Dollar Tree Inc <DLTR.O> reported a higher-than-expected jump in quarterly profit on Wednesday and boosted its full-year sales and earnings forecast, sending its shares up nearly 5 percent.

The company, which sells most of its merchandise for $1, said net income rose 51 percent to $56.9 million, or 63 cents per share, in the second quarter ended August 1 from $37.6 million, or 42 cents per share, a year earlier.

Analysts on average had been expecting profit of 54 cents per share, according to Reuters Estimates.

While other retailers have faltered in the recession, Dollar Tree has flourished as the downturn forces shoppers to seek out deals in its stores to stretch limited budgets.

The company has said customers are "trading down" from other retailers, searching for bargains on everything from food to beauty products and party supplies.

Dollar Tree, which operated 3,717 stores as of August 1, said quarterly sales rose 12 percent to $1.22 billion, while sales at stores open at least a year, a key retail gauge known as same-store sales, rose 6.8 percent.

For its third quarter, the company forecast sales of $1.19 billion to $1.23 billion, with a low-to-mid-single-digit rise in same-store sales. It expects earnings of 58 cents to 64 cents per share, while analysts had forecast 54 cents.

Dollar Tree now expects full-year sales of $5.09 billion to $5.19 billion, up from a prior outlook of $5.05 billion to $5.15 billion.

The company forecast full-year earnings of $3.10 to $3.25 a share, compared with a previous outlook of $2.75 to $2.90. Analysts on average were expecting $3.01.

Dollar Tree shares were up 4.9 percent to $50.24 in premarket trading.

(Reporting by Nicole Maestri; Editing by Maureen Bavdek and Lisa Von Ahn)