High-strength aluminum is used extensively in industry, with welding being a widely used fabrication method. This work focuses on welding of 6061-T651 aluminum and establishment of the hardness-tensile properties relationship in the heat-affected zone (HAZ) of a gas metal arc weld using 4043 filler material. Test welds were prepared from 12.7-mm-thick plate with a single-V weld preparation. Base plate temperatures were measured with an array of eight embedded thermocouples during welding, relating temperature to properties at intervals from the weld. Through-thickness slices 1.7 mm thick were removed, by electric discharge machining, from the plate parallel to the weld at 2-mm intervals and extending from the weld center-line to 40 mm into the HAZ and base plate. Tensile samples were prepared from these slices, and tensile properties and hardness values measured to establish a relationship between these two parameters. Both EQUOTIP (portable hardness tester) and Vickers microhardness measurements were conducted and related to tensile properties. Although a significant body of work exists relating tensile properties to hardness, no previous study was found that used this approach. Most work appeared to use cross-weld tensile tests, which only give the point of lowest strength. Sections of base plate material having a different thickness (31.75 mm) from that of the welded samples, and from a different source, were thermally aged to four hardness values and the hardness-tensile relationship was also established for this material. These results were compared with those of the HAZ samples; the results were found to fall within the scatter band of HAZ results.