A child's booster seat with detachable belt positioner car seat is provided. The booster seat can include a seat back that can be coupled and decoupled from a seat base. The seat base can include a primary seat base and a removable seat base. The removable seat base can be coupled and decoupled from the primary seat base and used as a booster seat for a child without the primary seat base. The primary seat base can include protrusions and/or recesses and the removable seat base can include recesses and/or bosses for aligning and/or coupling the removable seat base to the primary seat base. A pair of armrests can be adjustably coupled to the seat base, such as to the removable seat base. The seat base can further include one or more lap belt positioners. The seat back can include an adjustable headrest and one or more shoulder belt positioners.