No, the Maxi-Cosi Pria 85 cannot be used as a booster seat. The Maxi Cosi Pria 85 is a 2-in-1 convertible car seat, and it's designed to support a child in rear-facing modes and forward-facing. With that in mind, the Maxi-Cosi car seat holds babies up to 40 pounds rear-facing and children up to 85 pounds forward-facing with no booster seat mode.