An adjustable cycle-type seat post assembly comprises a tubular seat post which fits slidably within an upstanding tubular seat mast. The seat post has a cylindrical bore with a closed upper end and an open lower end. A mounting shaft has an elongated body comprising upper and lower head portions with an intermediate portion between them. The upper head portion has a piston with O-ring seals engaging the cylindrical bore and defining with the closed upper end a variable volume air chamber. The intermediate portion has an axially extending slot engaging a pin secured to the seat post to limit relative axial movement and prevent relative torsional movement. The mounting shaft is secured in the seat mast by axially spaced openings in the lower head, one of which is engaged with a water bottle connecting stud extending through the seat mast wall. Ribs flanking the openings align them with the water bottle stud to facilitate assembly. A bicycle pump valve at the upper end of the seat post enables pressurization of the air chamber. A quick-release clamp at the top of the seat mast holds the seat post at a selected, elevated position.