An adjustable seat post for bicycles has a control assembly, an installing tube assembly, a connecting tube assembly, and an adjustable element. The control assembly has a control-wheel and a curved-portion formed on an external surface of the control-wheel. The installing tube assembly is connected to the control assembly. The connecting tube assembly is movably mounted in the installing tube assembly and has a connecting tube. The connecting tube is mounted in the installing tube assembly. The adjustable element is assembled in the connecting tube assembly and has a trigger element formed in a bottom end of the adjustable element and abutting the control-wheel. The curved-portion is arc-shaped, such that the curved-portion abuts the trigger element at one point of contact. Therefore, the contact area between the control-wheel and the trigger element is minimized.