A belt tensioning system for an agricultural round baler, comprising: - a plurality of guide rollers which are parallel in orientation with respect to each other and separated from each other, - a baler belt which is configured in the form of an endless belt and which is positioned around the guide rollers, as well as - at least one tensioning device by means of which the baler belt can be put under tension, wherein at least one of the guide rollers is configured in the form of a drive roller by means of which the baler belt can be driven, wherein the guide rollers together define a run path around which the baler belt can run continuously, wherein, by means of the tensioning device, a distance between at least two guide rollers functioning as tensioning rollers (8, 9) can be varied, so that a length of a section of the run path of the baler belt which extends between the tensioning rollers (8, 9) can be varied, characterized by a self-contained tensioning unit which has at least two tensioning elements (12, 13) which are supported against each other as well as movable relative to each other, which each have at least one tensioning roller (8, 9), wherein the tensioning elements (12, 13) respectively cooperate with the tensioning device in a manner such that a tensile force exerted by means of the tensioning device acts in the opposite direction to the tensioning elements (12, 13), wherein the tensioning unit per se is independent of the tensile force of the tensioning device in an external balance of forces. - a plurality of guide rollers which are parallel in orientation with respect to each other and separated from each other, - a baler belt which is configured in the form of an endless belt and which is positioned around the guide rollers, as well as - at least one tensioning device by means of which the baler belt can be put under tension, wherein at least one of the guide rollers is configured in the form of a drive roller by means of which the baler belt can be driven, wherein the guide rollers together define a run path around which the baler belt can run continuously, wherein, by means of the tensioning device, a distance between at least two guide rollers functioning as tensioning rollers (8, 9) can be varied, so that a length of a section of the run path of the baler belt which extends between the tensioning rollers (8, 9) can be varied, characterized by a self-contained tensioning unit which has at least two tensioning elements (12, 13) which are supported against each other as well as movable relative to each other, which each have at least one tensioning roller (8, 9), wherein the tensioning elements (12, 13) respectively cooperate with the tensioning device in a manner such that a tensile force exerted by means of the tensioning device acts in the opposite direction to the tensioning elements (12, 13), wherein the tensioning unit per se is independent of the tensile force of the tensioning device in an external balance of forces. wherein at least one of the guide rollers is configured in the form of a drive roller by means of which the baler belt can be driven, wherein the guide rollers together define a run path around which the baler belt can run continuously, wherein, by means of the tensioning device, a distance between at least two guide rollers functioning as tensioning rollers (8, 9) can be varied, so that a length of a section of the run path of the baler belt which extends between the tensioning rollers (8, 9) can be varied, characterized by a self-contained tensioning unit which has at least two tensioning elements (12, 13) which are supported against each other as well as movable relative to each other, which each have at least one tensioning roller (8, 9), wherein the tensioning elements (12, 13) respectively cooperate with the tensioning device in a manner such that a tensile force exerted by means of the tensioning device acts in the opposite direction to the tensioning elements (12, 13), wherein the tensioning unit per se is independent of the tensile force of the tensioning device in an external balance of forces.