A sewing machine for forming running-stitch seams, comprises a needle (1) provided, proximate to its tip, with an open eye (2) and orientated so that its axis (1a) is substantially perpendicular to a surface (3) for supporting the item being sewn. The needle (1) is actuated with a reciprocating motion along its own axis (1a) passing cyclically through the supporting surface (3). A thread take-up wheel (5) is provided below the supporting surface (3) and is composed of a fixed part (6), with a magazine (8) for the taken-up thread, and a rotatable part (7), rotationally actuated about its own axis (5a). The rotatable wheel part (7) supports a lower looper (11) engaging the thread loop below the supporting surface (3) to load the thread (12) in the magazine (8) formed in the wheel fixed part (6). Pneumatic tensioning means (14,15,16) for the thread portion lying below the supporting surface are further provided.