A foldable stroller includes a stroller frame having two side frame portions, each of which includes a push rod, a leg rod, a pivot joint, and an armrest assembly including an armrest and an armrest folding device. The pivot joint has an upper joint member connected to the push rod, and a lower joint member connected to the leg rod. The armrest folding device includes: a connecting unit connected fixedly to the armrest, connected pivotally to the lower joint member, and having a guide slot formed therethrough; and a guide pin connected to the the upper joint member and extending through the guide slot. When the push rod is rotated relative to the leg rod, the guide pin moves between too opposite ends of the guide slot to thereby rotate the connecting unit relative to the lower leg rod.