The B9158 is a short road in south Aberdeenshire across the Dinnet Bridge, built in 1935 to cross the River Dee; the 1923 MoT map suggests there was a river crossing here beforehand. The road was originally unclassified but became the B9119 later in the 1930s before gaining its present number when the B9119 and A97 were swapped. As the lower B915x numbers were being used on the old sections of A9 around Aviemore in the 1980s, it can be presumed that this number was issued later.
At less than half a mile long, the road is one of the shortest rural B roads in Scotland. It follows a straight course through woodland from the crossroads on the A93 in Dinnet down to Dinnet Bridge over the River Dee. It ends on the far side of the river at a T-junction on the B976 (former A973).