A golf ball includes a core including a thermoplastic inner core and a thermoset outer core layer. The inner core has a surface hardness of about 40 to 80 Shore C and a center hardness of about 30 to 75 Shore C. The center hardness is less than the surface hardness to define a positive hardness gradient. A cover is formed over the core and includes an inner cover layer and an outer cover layer. The thermoplastic inner core includes a highly-neutralized ionomer formed from a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater. The outer core layer comprises a polybutadiene rubber and has a positive hardness gradient of less than 25 Shore C.