There is provided a frac adapter configured to couple a frac tree to a wellhead component. The frac adapter may couple the frac tree to a casing head without a tubing head, thereby enabling the well to be fractured before the tubing head is installed. As a result, the tubing head used in well production may be pressure-rated for production pressures rather than for fracing pressures. The frac adapter may be coupled to or integral with the frac tree. In addition, a union-nut coupling may be employed to quickly and easily assemble and disassemble the frac adapter from the wellhead component, such as the casing head. The union-nut coupling may further enable the components to be pressure-tested before the fracturing process is initiated.