An article of footwear is provided having a pressurized fluid-filled chamber in the sole structure having a continuous perimeter weld and lacking an inflation channel. The chamber may be formed from opposing polymer sheets, a continuous perimeter weld lacking a weld channel bonding the sheets and forming an interior void between the sheets, and a fluid disposed within the interior void exerting outward pressure on the sheets. A method for making the chamber can include placing a pair of polymer sheets in an opposing arrangement, pressing opposing perimeter weld portions of the sheets against each other to form an airtight seal along a perimeter region of an interior void, supplying a flow of pressurized air between the opposing sheets into the interior void, and, while maintaining a desired fluid pressure, welding the perimeter weld portions of the sheets to each other. A high-frequency welder for forming chamber is also provided.