A portable pet booster seat apparatus is provided which is adapted to be fitted securely on the seat of a vehicle. The booster seat includes a base, a back, a front and side portions that are all interconnected. A strap is provided that extends across the back and/or base of the includes a base portion which includes attachment fittings that are adapted to be fitted to the attachment brackets of a LATCH infant seat device. The booster seat apparatus may also include an additional strap for use in attaching the device to vehicle seats not fitted with the LATCH device. The portable pet booster seat apparatus may also include a removable and replaceable cover assembly. The booster seat apparatus is attached to the seat at or near the base of the booster seat, and thus has improved stability. It can also be more firmly attached to the vehicle seat and is less likely to move or shift during use.