Abstract: A helmet with an adjustable safety strap. The length of the strap used for securing the helmet can be adjusted by the wearer continuously, or progressively in stages of 1.5 mm, and fixed or released in any of the positions with only one hand, by way of a turn-lock fastener. The turn-lock fastener is rigidly incorporated in the helmet material and has cords which are guided from its two opposite sides along an inner side of the helmet to deflecting rollers, or deflecting points, on the helmet material, and then further guided therefrom along the strap to be tightened. The safety strap can be worn around the nape of the neck, or include a chin piece, among other manners of construction.