Jack in the Box to Buy Del Taco, Expanding Fast-Food Reach (1)

Jack in the Box Inc. agreed to buy Del Taco Restaurants Inc., marrying the offbeat chain with a fast-food competitor also popular with late-night diners. The burger chain’s shares slumped on the news.

Jack in the Box will pay $12.51 a share in cash for Del Taco, a Mexican-inspired food brand concentrated largely in the southwestern U.S., representing a 66% premium over the closing price on Dec. 3. The total transaction is valued at $575 million, including existing debt, according to a statement Monday.

The deal expands the reach of two chains toiling largely in the shadows of larger ...