A sunshade assembly includes a pair of spaced-apart rails, a roller unit, a screen, and a position-retaining unit. Each rail has opposite front and rear ends, and is formed with a track therealong. The roller unit is spaced apart from the front end, and includes an inner tube member and an outer tube member. The screen has a fixed end secured to the outer tube member, a free end, and opposite peripheral edge portions extending into the tracks. The position-retaining unit includes an annular spool, a rope, and a spiral spring. The rope extends along one of the rails, loops around the rear end, and passes through the track in the rail. The spiral spring is disposed in the spool, and has an inner end connected to the outer tube member and an outer end connected to the spool.