AA 6061 alloy and interstitial-free (IF) steel plates were joined by the friction stir welding (FSW) method, and the microstructure, mechanical properties, and biaxial stretch formability of the friction stir welded (FSWed) parts were investigated. The results indicate that the FSWed parts showed optimum tensile strength during FSW with the 0.4-mm offset position of the tool. The Fe4Al13 intermetallic compound formed in the defect-free intersection of AA 6061 and IF-steel plates during FSW. The hardness of the IF-steel part of the FSWed region increased almost 90% relative to its initial hardness of HV0.2 105. The tensile and yield strengths of FSWed regions were approximately 170 MPa and 145 MPa, respectively. According to the formability tests, the Erichsen Index (EI) of the IF-steel, AA 6061, and the FSWed samples were determined to be 2.9 mm, 1.9 mm, and 2.1 mm, respectively. The EI of the FSWed sample was almost the same as that of the AA 6061 alloy. However, it decreased compared with that of the IF-steel. The force at EI (F-EI) was approximately 1180 N for the FSWed condition. This value is approximately 70% higher than that of AA 6061 alloy.