An extendable folding bed frame includes a pair of first sub-frames having a pair of spaced apart and extendable longitudinal bars, each longitudinal bar having a first end and a second end. The opposing longitudinal bars are fixedly connected together by a plurality of lateral support bars. The pair of first sub-frames are pivotally connected together at corresponding longitudinal bar first ends by first pivotal connecting members. The pair of first sub-frames are pivotable from an open configuration whereby each first sub-frame is aligned longitudinally to form a generally rectangular co-planar support surface and a folded configuration whereby each first sub-frame is substantially overlapped and adjacent to each other. The folding bed frame also includes a connecting bracket for attaching additional bed frames to provide a larger mattress support surface.