The invention provides a convenient, lightweight and streamlined connecting device for attaching a leader or tippet to a fly-fishing line. The Fly Line Connecting Device is configured with an expandable tubular lattice that allows for an in-line connection to the fly line. The connection device has an integral tubular lattice sized with an internal diameter that is slightly smaller that the fly line and when compressively displaced, operates to expand the internal diameter to allow insertion of the fly line. Once the fly line is fully inserted, the tubular lattice is allowed to contract and elastically constrict onto and thereby grip the semi-resilient surface of the fly line. When the fly line and leader line are placed in tension through the connecting device, the tubular lattice acts to constrict down on the fly-line to resist separation. The connecting device of the invention is configured with a loop, eyelet, knot-trapping feature, or short tippet to allow for the convenient attachment of the leader line. Alternatively, the connecting device is permanently attached or integrally formed with a full-length leader. The preferred embodiment of the connecting device is injection molded in an intermediate form that is configured to be manually or machine cold-drawn to the final working dimensions and optimal physical properties.