The B42 is a cross-country B-road in south County Londonderry.
The road starts on the A6 in the Sperrin Mountains and heads roughly southwards, quickly starting to descend. At the bottom of the hill the Moyola River is crossed, after which the B42 continues across the fields into Tobermore. It reaches a T-junction on the B41 where it TOTSOs left, immediately reaching the A29, multiplexing with it between adjacent mini-roundabouts. Road signs claim that the B42 also multiplexes with the B41 - but that would be a useless multiplex.
Originally the B42 ended in Tobermore but it now continues on, leaving town along Magherafelt Road. The road then heads southeast along a surprisingly straight road across the fields, eventually reaching Magherafelt itself. The road enters town from the north along Rainey Street, ending at a roundabout on the B40 to the west of the town centre.