SACRAMENTO—California’s chief marijuana regulator on Thursday told a gathering of growers, government officials and would-be entrepreneurs that the state will have a regulatory structure in place to oversee a developing multibillion-dollar cannabis market by Jan. 1.

Lori Ajax, chief of the Bureau of Medical Cannabis Regulation, said state agencies will rely on a system of temporary licenses and grace periods to ensure commercial sales of recreational marijuana can start in compliance with the voter-approved Proposition 64.