Discontinuously reinforced aluminum matrix composites (AMCs) have been widely applied in structures of aerospace industry. Wide industrial applications of AMCs depend on effective joining methods, which are dependent on the use of a specific material and process. As a new solid-state welding technique, friction stir welding (FSW) has been attempted for joining the AMCs in last few years. However, few attentions have been paid to the effect of initial heat treatment tempers of the AMCs on the FSW joints. In this work, 6 mm thick SiC_p/2009Al composite plates in both soft (solution temper) and hard (natural aging temper) conditions were successfully friction stir welded at a rotation rate of 800 r/min and a welding speed of 100 mm/min (named as Sol-FSW and T4-FSW samples). In the nugget zone (NZ) of both samples, the grain size and the distribution of the coarse Al₂Cu phases were similar. In the heat affected zone, two low hardness zones (LHZs) were observed. LHZ I adjacent to the NZ had the lowest hardness. Both samples had the similar hardness in this zone. For the Sol-FSW sample, LHZ II far away from the NZ had a higher hardness and was closer to the NZ compared to that of the T4-FSW sample. The ultimate tensile strength of both the samples was similar and reached 83% of T4-tempered base metal. Both samples failed in LHZ I adjacent to the NZ due to the lowest hardness in this zone. This indicates that for the SiC_p/2009Al composite under solution temper it is possible to produce similar joints to that under natural aging temper using FSW technique. FSW of the composites under soft condition is beneficial to enlarging the welding process window and reducing the tool wear.